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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old August 1st, 2001, 22:16   #1
alex_tdi
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Default maximum MPG at what MPH?

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 [img]images/icons/wink.gif[/img]

Alex
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Old August 1st, 2001, 23:05   #2
VW Vet
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Default Re: maximum MPG at what MPH?

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.
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Old August 2nd, 2001, 06:18   #3
MOGolf
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Default Re: maximum MPG at what MPH?

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.
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Old August 2nd, 2001, 06:51   #4
Chris Bell
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Default Re: maximum MPG at what MPH?

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 ]
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Old August 2nd, 2001, 08:06   #5
GSXR600
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Default Re: maximum MPG at what MPH?

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. [img]images/icons/smile.gif[/img]
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Old August 2nd, 2001, 08:22   #6
green+blue
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Default Re: maximum MPG at what MPH?

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.
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Old August 2nd, 2001, 09:45   #7
jayb79
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Default Re: maximum MPG at what MPH?

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. [img]images/icons/grin.gif[/img]
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Old August 2nd, 2001, 18:39   #8
alex_tdi
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Default Re: maximum MPG at what MPH?

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"?
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Old August 2nd, 2001, 20:36   #9
inmba
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Default Re: maximum MPG at what MPH?

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.
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Old August 3rd, 2001, 16:41   #10
jetmann
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Default Re: maximum MPG at what MPH?

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...
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Old August 4th, 2001, 00:26   #11
SlowSpeed
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Default Re: maximum MPG at what MPH?

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?
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Old August 4th, 2001, 00:40   #12
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Default Re: maximum MPG at what MPH?

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.
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Old August 4th, 2001, 03:49   #13
fincoop
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Default Re: maximum MPG at what MPH?

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
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Old August 4th, 2001, 04:59   #14
Lug_Nut
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Default Re: maximum MPG at what MPH?

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.
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Old August 4th, 2001, 07:15   #15
SoTxBill
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Default Re: maximum MPG at what MPH?

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.
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