maximum MPG at what MPH?

alex_tdi

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Hi,

Here's a question that have always been on my mind.

We've all been told that as speed increases, the amount energy/HP required to maintain that speed increases dramatically. That is the reason why the federal gov't imposed a 55MPH speed limit until recently.

However, different cars have different drag coefficients. Drag affects MPG at high speeds. So at some point, the faster you go, your MPG will go down.

So, how does one calculate the optimum MPG?

At what point does the marginal increase in MPH causes the MPG to decrease?

Thanks


Alex
 

VW Vet

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I don't know any of the formulas for calculating the best speed/mpg for a given vehicle, but it has been my experience with all of the cars I have owned that around 35-40 mph yeilds the best mpg.
 

MOGolf

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The slowest speed you can drive in the highest gear is the optimal (though probably not always practical). For a modern automatic transmission vehicle, this should be stated as the slowest speed you can drive in the highest gear with torque converter locked up.

Obviously this varies by vehicle. In the latter case above, the last several cars I have had would have different speeds - 35, 40 and 52.

I haven't figured out my 01 Golf yet. With its "adaptive" transmission, I think it is still trying to figure out me.
 

RIP TDI

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Here are results of a test I did awhile back.

(Table won't display for some reason...just go to the link below andn go to the 5th post)

The test is further explained here.

[ August 02, 2001: Message edited by: Chris Bell ]
 

GSXR600

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I heard that if you keep the engine at 1900rpm max torque range at that speed (55mph) is when you get the best MPG. Please correct me if i am wrong.
 

green+blue

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The question should be,

AT WHAT RPM LOWEST FUEL USE,

It seems intuitive that with car in tallest gear fuel economy will be greatest at most efficient rmp. We have found just a bit over 2000, on flat ground that translates to about 62/63 mph with a stock 5 speed.

It also seems that if we could get a 6th like those lucky #$@!^&%# across the pond the increase in gearing would equal a corrosponding increase in mpgs (less increased wind resistance). I know, I could change the gears in the 5 speed, but don't feel like going to that much trouble.
 

jayb79

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If you take a look at the old post from Cris you can see that the lower the RPM the less fuel is used. It could be there is a break point though were the RPM is so low it is not efficient or good for the engine(or you would go nuts from moving so slow). My NB is happy to idle along in 5th gear at about 28 mph indicated, as long as the load is light(just myself in the car) and the grade is close to level(or down hill). I could go a long way on a gallon of fuel at that speed.
 

alex_tdi

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So the drag doesn't affect any of this?

If all we think about is RPM and fuel usage, then a semi-truck and a Golf with the same gearing should have the same MPG at the same speed?

Or does the fuel usage/RPM depend on the drag?

I'm not clear on the concept of varying fuel usage per load. Is the reasoning somewhere along the lines of "more fuel will be required to maintain a particular RPM if there is more load... therefore, more drag = more load = more fuel"?
 

inmba

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I think the original question is actually quite difficult to answer. If you are looking only at the effect of drag then there is some speed where aerodynamic drag becomes a greater factor than others. I have heard that aerodynamic drag does not become an issue until ~50MPH and increases exponentially from there. Total mass has little impact on MPG at a constant speed. Internal combustion engines are more efficient (energy/unit fuel) at lower RPM, so something like 50MPH in 5th gear is probably near real-world max MPG.

It is interesting to think about the hypothetical situation where infinite gear ranges exist. What would be the MPG if you could reach drag-limited top speed (full engine load) at the torque peak of 1900RPM? I have not driven a CVT but I have heard they stay around the torque peak for max MPG.
 

jetmann

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I think when Shell was doing the MPG record runs in the '60's and '70's, they'd run the car up to about 33-34MPH, then shut down and coast until they were going just fast enough to guarantee being able to restart the engine by popping the clutch.

Also, it was determined that the faster you got to coasting speed, the less net fuel you burned. BMW used to tell ppl in it's driving primer to "accelerate as quickly as possible, without losing traction, to achieve best mileage," and they cited the Shell runs in some of their ads...
 

SlowSpeed

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FWIW...

With ships - the fuel consumption increases as the cube of the speed. Anyone know about airplanes? The automobile has at least two resistances: aerodynamic, and the rolling resistance of the wheels. (Railroads, with a steel wheel rolling on steel rails have the lowest rolling resistance there.)

I remember reading in an old engineering text book that the optimum fuel consumption is obtained at the speed of maximum torque. So if had the 6-speed gear box, at the same road speed as the 5-speed, we'd probably have lower fuel consumption.

Re: post of VW Vet - when I was a kid, I remember an old timer telling that during WW2, the highway speed limits were down ~40 mph, at least in upstate NY. Anyone old enough to remember that?
 

GotDiesel?

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Pretty sure aerodynamic drag (and fuel consumption) increase at the square of velocity. To go 40% faster you'll double your fuel burn. (1.4 squared is roughly 2) 2x the speed costs you 4x the fuel, etc.

Internal friction and road friction also increase at the square of the speed if I recall my physics classes examples correctly.

You can get phenomenal mileage if you really want to drive that slow.

The reason I bought this car is that I wanted to go reasonably fast and still get decent mileage. It used to pain me to watch the fuel economy displayed on the trip computer just plummet if I sped up from 60 to 70. Now I just don't care because at 70 I'm still getting better than 50 mpg.
 

fincoop

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Aerodynamics don't start to come into play in a car until 40mph. The best mileage can be had at 40 in 3rd/4th (results pending) gear.

Optimal MPG is as close to peak torque as you can be, without going under, at your current speed. This is why it is recommended that your shifts dance around the 2000 mark.

CVT and Peak torque: Would certainly increase our top speeds, and probably be the most efficient at accelerating. Audi's CVT is a monster and is also very very fast.

Coop
 

Lug_Nut

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Aerodynamic drag comes into play at any speed above zero. The increase in aerodynamic drag increases as a square of the speed increase. Increase the speed by 20% (speed times 1.2)and you have increased the air drag by 44% (drag times 1.2 times 1.2). At low speeds the drag is low so even doubling it has little effect, but the rate of air drag increase remains constant.
Maximum economy is achieved at FULL LOAD at maximum torque in our TDI. That statement is misleading because the desire is not a high MPG but a low burn rate per unit of power. The TDI engine can produce about 72 hp at full load at 1900 rpm. Operating at half load will produce about 36 hp at 1900 rpm. Operating at 36 hp will use slightly more than half the fuel used at full load, 72 hp.
At partial load, where most of our operation occurs, the desire is not specific consumption per hour per unit of power, but is distance per unit of fuel consumed. Maximum distance per volume of fuel used occurs in the highest gear at the lowest air drag, about 35 to 40 mph in the TDI.
 

SoTxBill

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last two comments i believe are right on the money... if we had a continuosly varible transmission,, we would see max mpg at max torque...minus air drag, it could be slightly lower due to less air drag...
but since we have fixed gears the air drag is the monster we are fighting..

also theretically, with a cv trans we could achieve max speed at peak torque... not peak hp... but again with fixed gears, it doesent work that way..

in later years manufactors have tried very very hard in all cars to make max torque right at legal speed limits for a reason...

that is,, max mpg at that speed...

thats why cv tranmissions are a neat in thery.. they always keep the engine at max torque which is where its' mosts powerful...

actually shifting should not be done at max torque but slightly above so that the actual driving rpms are kept at closer to max torque for best power. dont shift there, drive there.. so shifting at 400 below to 400 above would be a good plan. otr allison transmissions are actually program to do this to keep the mpg and power up at all times. shifting at the max torque forces the next gear to be way under and very inefficient till the rpm comes back up again.
 

czego82

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I'm not sure how many of you can beat that but recently i was driving back home, trick was that i was going 70mph about 5feet behind a semi truck. My car did about 80mpg. Now how can u bet that???
 

Lug_Nut

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70 mph is 103 feet per second. At a five foot interval the time gap is .05 seconds. How much can a semi truck slow in .05 seconds? The actual difference in closing speed will be so slight that a bent hood will be about the extent of your damage upon impact with the truck. It is the next impact with what ever it was that made the truck slam on the brakes that you won't have time to worry about.
 

alex_tdi

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OK guys,

Thanks for all the valuable info. I've always been a fan of the CVT, in theory at least, and have been waiting for a car maker to sell it here in the U.S.

Alas, Honda is the only car maker who sells them, in the Civics, and after one accident too many in a Civic, I've decided never to buy another Civic again.

Hopefully, VW will start selling a CVT mated to a TDI when I'm ready to buy another VW


Oh, we all know that it's probably not a good idea to stick so close behind a semi truck, but as we've seen, exactly how far is debatable. I for one is happy to stay as far away from those monsters as I can.
 

JazzMan

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I might be able to shed some light on the power calculation issue.

Starting with the basic equation:
Power = Force * Velocity
you end up with this after adding in the force components.

P = V(Fr*m*g + (1/2)*rho*V^2*Af*Cd + m*g*sin(theta))

where:
P = power requirement (in kW)
Fr = rolling resistance coefficient (unitless)
m = mass (kg)
g = gravity (9.8 m/s^2)
rho = fluid density (in this case, air)(kg/m^3)
V = velocity (m/s)
Af = frontal area (m^2)
Cd = drag coefficient (unitless)
theta = angle of climb/decent (degrees from horizontal) (positive for ascent, negative for descent)

I hope I didn't give anyone a headache.
 

GotDiesel?

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5 feet behind a semi... sure, be my guest.

I'm not sure what'd be worse--the rocks coming at your windshield from the truck ahead or your head going through the windshield when the truck stops faster than your reaction time.

Is your life/car really worth the meager fuel savings??

But since you asked, here's how you beat that... put a trailer hitch on the front of your car and just hook it up to something to pull it. Infinite MPG.
 

jetmann

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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by czego82:
I'm not sure how many of you can beat that but recently i was driving back home, trick was that i was going 70mph about 5feet behind a semi truck. My car did about 80mpg. Now how can u bet that?<HR></BLOCKQUOTE>

Did that once: Indy to Fort Wayne in a BMW, 112 miles, 1.9 gal., 58.4MPG. After I thought about it, I decided that 158MPG would not have been enough for the danger and stupidity which that kind of driving entails. That was 15 years ago. I'm still alive, and the lesson is still sticking
 

alex_tdi

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Hey JazzMan,

How about a Cliff Note's version?

Since you also drive a '01 TDI, how about determining the max MPH at MPG?

Thanks.

Alex
 

jetmann

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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by Jonathan Bartlett:
70 mph is 103 feet per second. At a five foot interval the time gap is .05 seconds. How much can a semi truck slow in .05 seconds? The actual difference in closing speed will be so slight that a bent hood will be about the extent of your damage upon impact with the truck. It is the next impact with what ever it was that made the truck slam on the brakes that you won't have time to worry about.<HR></BLOCKQUOTE>That's true, J.B., but you're also assuming an instantaneous reaction on the part of the draftee, and assuming the truck doesn't simply hit something...A deer or large dog would be sufficient, I suspect, to remove a .05sec disaster margin. At any rate, I'd draft a semi if there were sufficient reason to do so, like an hypothetical situation in which I was on my last gallon of fuel and 60 miles away from the nearest petrol store, and running on a schedule such that the delay from running out of diesel would compromise someone's life or property (hafta be a lot of property to be worth my life ;-)

Since I try not to get into situations like the preceeding, I doubt I'll be tailgating any semis in the forseeable future. If you choose to do so, please be careful!
 

JazzMan

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Right, VW Vet. If you take the first V (the one before the opening parenthesis) and multiply it out through the equation to get rid of the parenthesis, your drag-force portion becomes V^3, the cube of the speed. With these cars (the 5-speed specifically since I have no experience with the automatics) 60mph in 5th is about 2k rpm, where the engine is likely most efficient, and this is the speed where aerodynamics will begin to take affect. For most vehicles, aerodynamic drag is negligible below about 50mph. This is even more so for us since the coefficient of drag for our vehicles is relatively good ('01 Golf = 0.31, '01 Jetta = 0.30, '01 NB = 0.38). Therefore, by scientific evaluation (and a considerable amount of guessing
) 60mph will likely be the best MPG speed.
 

green+blue

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The original question was what is best mph for mpg.

IMHO the only 'fair' way to consider this question is in 'my' car (constant) which speed (intermed. variable) should i drive to achive highest milage (dependent variable).
Speed is intermediate because rpms (independent variable) and those factors listed below determine speed.

We can correct for many variables by assuming level ground. (None of this "I get great milage downhill". Hint: shut off the engine and carefully choose your route so that all of your trips can be downhill, carry a strong friend to push if you get stuck.) Assume same car, weather wind conditions.(Yeah, my Jetta gets more mpgs than that Moving Van traveling at the same speed.)

What we are left with is that you WILL get best mpgs at about the peak torque speed for the engine (1950 rpm in my manual) in the most effient (5th) gear. Sure I might burn less fuel per minute at 28 mph, but if it takes me twice as long to get there I have to burn less than half at that lower speed to get better mpgs.

Taller gears have only two very minor penaltys on mpgs, you should have slightly more rolling resistance because your engine is doing a tiny bit more work to spin everything faster, and wind resistance increases with speed a minicule amount (I have yet to experience the 'compressability of air nearing Mach in my Jetta.)

So effectivley if you go 10 or 20 percent faster because of taller gears and get there 10 or 20 percent sooner you burn 10 or 20 percent less fuel.
 

VW Vet

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Keep this in mind: Whatever your car gets for mpg @ 60 mph, it will get better mpg @ 45 mph. It all has to do with drag. That is what a car has to overcome most of all.
A loose example: a plane which flies @ 80 mph with a 75 hp engine will require 300 hp to go 150 mph. (not an exact example, but you can see what I mean)
 

green+blue

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Drag? this is a Jetta not a Tri-Pacer. Is the combined increase in aerodynamic/mechanical drag at 60 mph 33% more than at 45 mph. It would have to be because if you can run at 60 mph in 5th at 2000 or so rpm, which green does daily, you will drive 33% fewer minutes and unless you just want to sit in the parking lot idling, you will consume less fuel getting there. Sure at some higher speed you will balance increased resistance with fuel saved by getting there sooner but I think that will be at a bit higher spped than 45/60.
 

HDIesel

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green and blue:

are you sure you are not thinking mph as in hours?

You all should be happy with your aero drags(well, maybe not beetle owners). I drive a citroen minivan with HDI and my mpg is about 42US MPG half loaded. I usually drive @70MPH
 

SteveS

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The answer is 52 MPH -- which is 1900 rpm in my 00 automatic NB TDI. Best mileage has been 59.6 MPG --remember, this is an automatic.
Now for applied P = V(Fr*m*g + (1/2)*rho*V^2*Af*Cd + m*g*sin)
I've played with this on several economy runs on the wide open, flat highways which literally begin where I live and extend to the east for several hundred wide open miles.
On these economy runs, I am able to fuel and immediately start the open road economy test. Under very conservative driving, this TDI easily surpasses 50 MPG, but I like speed far too much to drive like this, so I consistently get lower mileage, but great fun driving like a madman.
 
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