"Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

boyelectric

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"Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

I keep seeing people referring to maximizing your "mileage" or "MPG" by making sure you fill up your tank 'all the way' using the vent trick.

Clarification: Mileage (MPG) will NOT increase by adding more fuel or less fuel. The measurement is specifically independent of total quantity of fuel (miles PER gallon).

Conversely, "miles per tank" is another way of saying "Mile range." This is a figure that is entirely dependent on how much total fuel you have. Neither of these are MPG!

Further clarification: to better assess MPG, one must figure the miles traveled across several consecutive tanks of fuel... this eliminates the issues of how much or little it was filled up at each fueling stop. The more consecutive fillups in a row, juxtaposed against the total miles travelled, the more accurate your numbers.

-a-
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by boyelectric:
...Further clarification: to better assess MPG, one must figure the miles traveled across several consecutive tanks of fuel... this eliminates the issues of how much or little it was filled up at each fueling stop. The more consecutive fillups in a row, juxtaposed against the total miles travelled, the more accurate your numbers.

-a-
<font size="2" face="Verdana, Helvetica, sans-serif">Perhaps... but there ARE those of us who use the same pump, parked in the same spot, and vent each and every tank to the brim. IMHO, that is enough of a "method" to figure accurate mileage with each individual tank.

I prefer to compare my mileage to the conditions I was driving under on a tank-by-tank basis. So far, I've got 53k miles worth of data, all vented tanks. I keep notes about tanks that involve unique circumstances, i.e., high speed trips, cruise set at 60 mph, winterized B20, etc.

Not to knock your post, but I think it's misleading to suggest that one "must" average figures over multiple tanks to achieve an accurate figure.
 

boyelectric

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

There are so many variables when it comes to each fill-up, including the coefficient of expansion of the fuel based on temperature, (5 gallons in the car @ 75deg, 10 from the pump @ 50deg), the angle of the car and how the fuel is hitting the baffles on the inside of the tank and what air gaps are left, etc. etc.

It is not very good scientific process to base any conclusions on one datum (one fill up). I agree that with careful, consistent practice at the same pump, and with other variable attended to, one can much more precise, but not as close as over multiple, consecutive fillups.
 

AutoDiesel

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

You are very correct!

Too many people are concerned with individual tank fills. It is fun to track mpg's when you have a vehicle that is capable of very good results like our TDi's but the real figure to shoot for is your lifetime average.
I haven't get very good records lately but I do have the figures for the last three tanks.
1) 430 miles / 10 gallons
2) 339 miles / 8.6 gallons
3) 346 miles / 9 gallons
= 1115 mies / 27.6 gallons = 40.39855 mpg

Not bad for 75 to 80 percent city driving.
And a automatic with .184's, TuningBox, K&N panel, and a 2.0L MAF. The 2.0L MAF has lower our mileage about 2 to 3 mpg's out on the highway but our city mileage seems to be better. We used to get 34 to 36 mostly in the city but I haven't seen less than 37.5 mpg for a long time.
I'm happy so far!
 

boyelectric

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by AutoDiesel:
... the real figure to shoot for is your lifetime average.
<font size="2" face="Verdana, Helvetica, sans-serif">Yep, or even an average for a cross-country trip, where you have very similar driving habits across several tanks.... let's see, with 4 tanks, I've gone from the Pacific ocean to the Atlantic!
 

RiceEater

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

A lot of posts proudly declare <60 mpg and they say highway but they accomplished this by traveling <45 mph. There are just a few people who scientifically qualify their data with % city speed and highway speed data. There are other variables such as altitude, relative humility, temperature, etc. Most the numbers posted are just worthless drivel.
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Damn, hit the wrong button, sorry.

[ August 13, 2002, 14:27: Message edited by: BongoBrains ]
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by boyelectric:
There are so many variables when it comes to each fill-up, including the coefficient of expansion of the fuel based on temperature, (5 gallons in the car @ 75deg, 10 from the pump @ 50deg), the angle of the car and how the fuel is hitting the baffles on the inside of the tank and what air gaps are left, etc. etc.

It is not very good scientific process to base any conclusions on one datum (one fill up). I agree that with careful, consistent practice at the same pump, and with other variable attended to, one can much more precise, but not as close as over multiple, consecutive fillups.
<font size="2" face="Verdana, Helvetica, sans-serif">I guess that all depends on the "data" you are trying to collect. If you are after lifetime mpg figures, sure, I can give you those. If you are after "900 mile club" stats, I can give you those too. It's really not that difficult. You pump in a full tank of fuel, drive till the next fillup, and repeat the process. The fuel left in the tank (after 900 miles) is minimal, and any temp difference from that previous fillup is really not very significant. The 'miles covered' figure is possibly 'contaminated' by gauge error/tire wear, but that would be the case using either method. The amount of fuel pumped in is a given constant, unless the particular pump has been recalibrated between fillups.

To me, it's kinda like saying, "the temperature outside averages 65 degrees during August". That does nothing to give me a feel for how hot it is when I go outside today. Right now, with the sun out, it's a damn humid 90 degrees. Tonight, it might dip to 70.

I prefer to have data that reflects a smaller set of indicators. Getting lifetime data is still possible, but that isn't the data I'm after. I truly believe that my methods are consistent enough to supply accurate data. I'm not too concerned with trying to convert others to do the same.

Again, I'm not trying to start a war here, but as most people know, the 'data game' can often be played to support whatever findings a person wishes to get. I want data that shows how many mpg I got on my last tank, and do everything I can within reason to get an accurate number for that. Anything beyond that is "splitting hairs" IMO.
 

VW Racer

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by boyelectric:
I keep seeing people referring to maximizing your "mileage" or "MPG" by making sure you fill up your tank 'all the way' using the vent trick.

Clarification: Mileage (MPG) will NOT increase by adding more fuel or less fuel. The measurement is specifically independent of total quantity of fuel (miles PER gallon).

Conversely, "miles per tank" is another way of saying "Mile range." This is a figure that is entirely dependent on how much total fuel you have. Neither of these are MPG!

Further clarification: to better assess MPG, one must figure the miles traveled across several consecutive tanks of fuel... this eliminates the issues of how much or little it was filled up at each fueling stop. The more consecutive fillups in a row, juxtaposed against the total miles travelled, the more accurate your numbers.

-a-
<font size="2" face="Verdana, Helvetica, sans-serif">Not to start a food fight here, boyelectric, but it is just as rational to talk about maximizing your fuel "mileage" when referring to a tank of diesel as it is when specifying MPG. Your implication that "mileage" refers exclusively to "MPG" is too narrow. Don't believe me? Well, it's not just my opinion. Type the word "mileage" in at www.wordsmyth.net and you will get four different definitions of the word. And "miles per unit fuel" is only one of them. Besides, a tank of diesel is as rational a "unit of fuel" as any. We accommodate liters and Imperial and American gallons here with minimal confusion, mixing in miles and kilometers along the way. Surely we can discuss miles per tank without overtaxing the imagination.

"Mile Range"? How about just "range" - the standard term when discussing the distance a vehicle can travel on the fuel it can carry.

Finally, IMO, your contention that one must measure fuel consumption over several tanks of fuel in order "to better assess MPG" is useful only for calculating long-term average MPG. It tells me nothing about my city, freeway or mountain mileage. Any time I accurately refill the tank, I can derive MPG accurate to the least significant digit for my current driving conditions.
 

jck66

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by boyelectric:
There are so many variables when it comes to each fill-up, including the coefficient of expansion of the fuel based on temperature, (5 gallons in the car @ 75deg, 10 from the pump @ 50deg)
<font size="2" face="Verdana, Helvetica, sans-serif">The coefficient of thermal expansion of diesel is small (0.00046/degF). This means that 10 gallons of fuel at 50 degF will turn into 10.1 gallons at 75 degF. It's really not enough to worry about.

Otherwise, you are right on the mark.

PS I got the number from here .
 

boyelectric

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Okay, I'll concede the "Mileage" term might have broader implications... that just means that people ought to be certain to what it is that they are referring... expecially the difference between MPG and range.

Here's what I am saying stated another way:
1) I don't inherently trust the data point of one fill-up. I have seen many people report "getting 60MPG" one week, and "45MPG" the next (or some example similar). There are too many variables around one fill up. Now if you are reporting 51.5MPG on one fill-up, 50.3MPG on the next, 52.8MPG on the next, etc, etc., and are careful about your variables, I might tend to think your data is more accurate. I take exception to people who get wildly fluctuating mileage and then attribute all manner of things to explain the fluctuations, when the first and most obvious explanations are around the variables of filling up.

2) Adding more fuel to the tank (ie: vent trick) doesn't change MPG one iota.

BTW: THANK YOU for the coefficient of diesel expansion! I was looking for that a long while ago and could not find it. I even asked my cousin who worked for Detroit Diesel, and he didn't know.

-a-
 

Derrel H Green

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"



Quoting exactly from the aforementioned website directly:

THERMAL EXPANSION
Like all liquids, diesel fuel slightly expands in volume as its temperature increases. The coefficient of thermal expansion measures the rate of the expansion. A typical value of the coefficient of thermal expansion for diesel fuel is 0.00046 per degree Fahrenheit. Using this value, 1.000 gallon of diesel fuel at 20°F will expand to 1.037 gal-lons at 100°F.

Is that actually 0.00046 X 80 (degrees) = 0.0368, or 3.68 percent?


Maybe I'm splitting hairs? Do we ever see anywhere near that type of temperature spread? Not here in mild California.




 

pedaller

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by WVWSP61:

Is that actually 0.00046 X 80 (degrees) = 0.0368, or 3.68 percent?


Maybe I'm splitting hairs?


Multiply by 1.00046 for each degree.
1.00046 to the power 80 = 1.0374767...

The fuel tanks underground can be significantly cooler than ambient, but I doubt we'll ever see 80 F difference.
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

So, (to get this straight), to accurately figure my mileage, I should have:

A temp reading from the underground storage tank.
A temp reading from the fuel that remains in my tank. A precise numeric value for the amount of fuel in my tank prior to fillup. A mathematical formula to determine the coefficient of expansion in relation to the existing fuel vs. the pumped fuel. A consistent angle at which the car has to be positioned in order to negate any chance of air pockets in the fuel tank when filling. Readings for ambient air temp at the time of fillup AND during the period of fuel use. Barometric readings during that same time period. Wind direction figures with GPS tracking coordinates to determine true aerodynamic drag figures. Fully charted MPH readings during the full use of tank. Time spent "at idle" also documented and figured into Fuel burn statistics. Etc. etc. etc... ***AND THEN, only average my figures over multiple tanks...***

Do yourselves a favor, if you want to get an accurate mpg reading, simply vent each tank to the brim, parked at the same pump, in the same spot.

Every other factor here will have an "influence" on your readings REGARDLESS of which data indicators you choose to use. Fuel temp induced expansion is going to effect everyone. The "data" is going to be influenced in EITHER CASE.

There are only a few things a person can do to aquire "accurate data". It begins with the three simple steps I mentioned in post #2. The rest of this debate is quite anal.

To each, his own, but please don't come on here with a *condescending* post that "enlightens" us with inaccurate "facts".

Sorry if I've ruffled feathers, but "diplomacy" didn't seem to have any effect...
 

GeWilli

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

I've read the first post - the second post and the last post

Forget temps

Forget everything else.

Folks like BongoBrains and I are venting - everytime #1

Filling at the same pump 99% of the time (even from the same side I'd wager -true for me)

I even generally fill at the same time of day (once every 6 or so days in the evening)

underground temps ambient temps - screw it - it'll be relatively close enough to the last time I was there that hte change will be gradual enough.

Figgure this brainacs. Underground fuel temp will change by about 5 degrees or so (with a properly burried tank) in the course of the year.

If you are fillin up empty at roughly the same time using the same pump to the same visual level (venting) then where is the problem.

Now for all yall super anal Folks.

Plot your MPG (or L/100km) on a chart - then run a mulit point fit line for it. I run a 5 or a 10 point fit line. Makes brain dead simple the consistency. And Lemme tell you.

IF you don't drive much (boundless) and don't vent (boundless) your data is pretty much only worth a 5 to 10 point analysis. If you only fill up in the winter 5 times - how the heck are you going to get anything meaningful out of keeping track of the mileage!?

-

NOW one thing important not to miss in this whole thing.

Keeping track of your mileage allows you to monitor performance and functionality of the car.

If your mileage starts tankning and you haven't changed driving habits . . . well I'll give ya a clue - something is wrong . . .

I'm not always known for diplomacy and politeness (I'll leave that for BongoBrains) facts.

Lets deal with facts.

If you can make sense of you title in a way that makes sense in the broad picture - that would be miraculous
 

pedaller

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by GeWilli:
I've read the first post - the second post and the last post

Forget temps

Forget everything else.

Folks like BongoBrains and I are venting - everytime #1


Exactly. Consistency is the key.

Regarding storage tank temp, I was answering some specific questions, not suggesting that we get hung up on it. The pumps are normally temp corrected anyway, so you get the same BTUs pumped per litre/gallon on the meter.

I get consistent mileage figures from tank to tank due to consistent filling. When there are variations of 2-3 mpg when all else is equal, there is a reason (like the sticking hand brake cable I recently replaced).
 

Srikanth

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Isn't the diesel volume pumped out from the pump corrected to 15c or some thing. I have seen the signs at most petrol filling stations, from what I can recall.
 

GeWilli

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by NB TDI:
well i think that if you fill up your car with 5 gallon drive until empty.. and the fill it up 15 gallons drive the same road until empty.. you'd get better mpg on the gallon tank because less weight
<font size="2" face="Verdana, Helvetica, sans-serif">well sure the mileage AVERAGE might be 'higher' but only slightly and wouldn't you also have to fill up 3 times more often if you only have 5 gallons instead of 15 gallons in there?

And then what is the price? Cause once the 5 gallon person has filled up 2x then the 15 gallon person is getting the same mpg for the same last 5 gallons . . . .

I'd hate to be the 5 gallon person flawed logic.

Time costs money - fillin up that often also entails stopping and starting and slowing and accel more than the 15 gallon person so maybe that might even negate the fractional MPG gain you'd have to having a few pounds lighter load . . .
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

...and keep in mind that we've now jumped from "how to figure your mileage accurately", to, "how to get better mileage from the same amount of fuel". If we keep going on like this, we might open up "Pandora's Box", and be forced to calculate every dynamic of fuel mileage that anyone can add to the mix. I'll have to add a tow hitch and trailer to haul around a SuperComputer to calculate numbers so I can document my mileage in the little black book in my glove compartment. That's surely going to wreak havoc on my mileage figures... And to think that my little solar powered calculator was doing the job... What was I thinking?
 

boyelectric

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

I don't care how many times you do it once...
, data are more accurate with more data points. All I am saying, regardless of what anyone does to affect their mileage, one tank of fuel, involving one action of filling up, is only one data point.

One data point is only one data point.

Of course, one can get pretty good with the process, and probably get some pretty consistent results... yea to you. Once again the target of this is the person who is getting wildly fluctuating mileage from fill-up to fill-up, and attributes it wholly to factors that may have absolutely nothing to do with anything. If you change a single variable once every 3 tanks or something, your data will be much more accurate!

Didn't anyone take a science course in gradeschool?

-a-

Also, can anyone say that they have ever gotten all the air out of the tank when filling? Ever heard the fuel "slosh" around when you leave the pump?

http://forums.tdiclub.com/NonCGI/ultimatebb.php?ubb=get_topic;f=3;t=001804#000016

The tank is irregularly shaped, diesel foams, we have a partial venting system, etc. Calculating over multiple tanks cuts these errors down to a minimum.

[ August 15, 2002, 14:50: Message edited by: boyelectric ]
 

jjvincent

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

When I was doing endurance car racing we were always concerned about fuel consumption. This is how we would check it:

1. Drain the tank.
2. Fill tank with 10 gallons of fuel.
3. Run the car on the track for 30 minutes.
4. Drain the tank and measure what's left.

We measured fuel consumption by gallons per hour. The races were all timed(not laps), so that was important. For fun, I would calculate the fuel mileage also. Amazingly, the fuel consumption didn't change too much depending on the track. Road America gave the worst and most street courses gave the best. The difference was about 2 miles per gallon.

For racing, knowing how long you can go on a tank of fuel is quite important. If you're off too much you'll run out. We never ran out of fuel so the fuel calculations were accurate enough for us. We did cut it close sometimes. At Sebring we were cutting it close. We crossed the finish line at 3hr.59min.55sec., now we needed to go another lap. The car made it across the finish line but quit on the cool off lap. Just like the calculations said it would. BTW: we won also.

One other way to check fuel mileage is to equip your car with a PI system. Put on 2 fuel counters and a wheel speed sensor. Now you have realtime data.
 

boyelectric

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by jjvincent:
When I was doing endurance car racing we were always concerned about fuel consumption. This is how we would check it:

1. Drain the tank.
2. Fill tank with 10 gallons of fuel.
3. Run the car on the track for 30 minutes.
4. Drain the tank and measure what's left.

We measured fuel consumption by gallons per hour. The races were all timed(not laps), so that was important. For fun, I would calculate the fuel mileage also. Amazingly, the fuel consumption didn't change too much depending on the track. Road America gave the worst and most street courses gave the best. The difference was about 2 miles per gallon.

For racing, knowing how long you can go on a tank of fuel is quite important. If you're off too much you'll run out. We never ran out of fuel so the fuel calculations were accurate enough for us. We did cut it close sometimes. At Sebring we were cutting it close. We crossed the finish line at 3hr.59min.55sec., now we needed to go another lap. The car made it across the finish line but quit on the cool off lap. Just like the calculations said it would. BTW: we won also.

One other way to check fuel mileage is to equip your car with a PI system. Put on 2 fuel counters and a wheel speed sensor. Now you have realtime data.
<font size="2" face="Verdana, Helvetica, sans-serif">I like this guy. Now we're talking!
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

If someone is getting wildly fluctuating figures between tanks, STEP ONE is...

Tank Filling Consistancy!!! Fill the tank the same way each and every time, minimizing any chance of variation between fillups. This is simple stuff right??? Ok, if I haven't lost you yet, that was what I wrote at post #2 of this thread.

Now, REGARDLESS of how many indicators you choose to use, or how many tanks you want to average, or if you have had any schooling in your life; the DATA is going to be MORE ACCURATE if you have minimized the variables under YOUR CONTROL. Again, see post #2.

Your position about multiple indicators is only "holding water" if you first get ACCURATE INDICATORS. If you can't grasp that simple fact, I'm absolutely wasting my time here, except for the potential amusement of others...

Now, I'm sorry if you have issues with people who don't vent their tanks, and / or post wild data about their mileage figures. Go ahead and school them with your brilliance. But there are those of us out there that can figure this stuff out without your help.

And for what it's worth, I work with data acquisition, data entry, and the variations between indicators every working day. Thanks for your input...
 

GotDiesel?

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

All this would be virtually moot if VW had just put the darn MFA in our cars like any halfway intelligent product manager should have.
 

boyelectric

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

A tank is an irregularly shaped vessel, it is not a pyrex flask... when it is full, even after venting, it sloshes... how do you guarantee that there is the same amount air each time?

From what I know of fuel tanks, their shape, specifically the tops of them, are unevenly corrugated (for structural strength). An uneven top leaves air gaps.

If I am wrong about the shape of the insides of the tank, please, someone correct me... I' be happy to retract this point.
 

BongoBrains

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

How many consecutive tanks have you vented to the brim, at the same pump, parked in the same spot??? What are your figures??? I've got mine, and they are damned consistant. Your theory is being challenged by those of us who actually practice a consitant method, and have the data documented to support our position. ***???

It's amazing how in the past, I've been attacked about "venting" by those who swear against it's worth, yet they have absolutely nothing to go on but their theory. They have even openly admitted that they have never vented a tank, yet they want to "school" others on the process to determine mileage data.

I've stated it before, and will again, I don't really care if you "vent" or not. I've got my data, and I'll put it up against anyone who wants to prove it worthless vs. theirs.
 

jjvincent

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Jack up the RR of the car while you're filling it. If there are any air pockets in the tank, they might come out.
We used to watch the Camaro's do this when they would come in for a fuel stop. With a 944 we could only get in an extra 0.2 gallon.
 

GoFaster

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Re: "Mileage" vs. "MPG" vs. "Mile range" vs. "Miles per tank"

Originally posted by jjvincent:
Amazingly, the fuel consumption didn't change too much depending on the track. Road America gave the worst and most street courses gave the best. The difference was about 2 miles per gallon.

For racing, knowing how long you can go on a tank of fuel is quite important. If you're off too much you'll run out. We never ran out of fuel so the fuel calculations were accurate enough for us. We did cut it close sometimes. At Sebring we were cutting it close. We crossed the finish line at 3hr.59min.55sec., now we needed to go another lap. The car made it across the finish line but quit on the cool off lap. Just like the calculations said it would. BTW: we won also.
<font size="2" face="Verdana, Helvetica, sans-serif">heh heh, your observations about Road America agree with mine ... throttle is pinned at max for most of every lap there.

With my race Yamaha FZR400, I stick a ruler into the fuel tank and measure the depth ... easy, because of the shape of the fuel tank on that bike. At Shannonville, it uses an inch every 10 minutes of on-track time. At Road America, given that the throttle was pinned for about 90% of every lap ... I ran out of gas on the last lap of my first practice session there. Oops.
 
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