[Info-vax] Off-topic follow-up, now with actual data - mileage vs. speed

[alanfeldman48 at gmail.com]

A while back, I think twice even, discussion of gas mileage came up for some 
reason. Many here claimed higher speeds get better mileage. I claimed that 
going at low RPMs in the highest gear gets the best mileage (maybe about 50 
mph). I was _censured_ for this (I was going to say "mildly excoriated", but 
that's kind of like small times big). So I think it's only fair for me to 
respond now that I have some actual data. (And more external references.)

I finally have a car with a mileage meter. I drove it on flat, slightly 
inclined stretch of an interstate highway about 0.3 miles long. 

Results:

Mileage in MPG while going slightly uphill on a flat bridge 0.3 miles long 
using the cruise control. First column is speed in mph, 2nd and 3rd, mileage in 
miles per gallon.

55	29.17 +/- .93	29.09 +/- .91
60
65	26.9 +/- 2.1
70	26.2 +/- 2.1
75	23.9 +/- 1.2	23.9 +/- 1.4

The display is updated every 2 seconds. It is digital and reads with a 
precision of .1 MPG. (Not necessarily accuracy. I mean I get readings like 
24.2, 25.0, etc. Even if it's not accurate to that level, I'd still expect 
higher mileage would give a higher reading.) I entered them into my hp 32S 
calculator and calculated the averages and standard deviations of each data 
set. Above are my results. I made two runs at 55, two runs at 75, and one each 
at 65 and 70.

I made similar runs on previous dates, but only kind of averaged the readings 
in my head. I *always* clearly got better mileage at lower speeds. 

Consumer Reports published a similar test using 5 different cars going at 
speeds of 55, 65, and 75 mph in their August 2013 issue. I won't type in the 
entire table, but the MPG drops from 55-75mph were 29%, 27%, 27%, 32%, and 27%. 
(Actually, such a table is at one of the links below.)

It's clear: 55 mph gets much better mileage than 75 mph.

Why is this? It's because drag is increased at higher speeds. The work needed 
to move the car over a fixed distance is the average force times the distance. 
This "work" (in the physics sense) is the amount of energy needed from the 
engine. Since the distance is fixed and the force (drag) goes up with speed, so 
does the work (the amount of energy) needed to keep the car moving increases. 
Some claim the engine gets more efficient at higher RPMs. That may well be 
true, but it's not enough to overcome the increased drag.

I once went on a road trip from CA to CO with a couple of friends (c. 1982). I 
had the same argument with one of them. He said mileage would increase with 
speed. I said the mileage would vary inversely as the speed. I forget what 
the speeds were. Maybe 55 vs. 65. Anyway, we got better mileage at the lower 
speed, but the decrease in mileage from going faster was less than I predicted, 
showing there may well be higher efficiency at higher RPMs. Again, it's not 
enough to overcome the increased drag and higher speeds. (We did the old 
fashioned fill up the tank, write down how many miles you drive, fill up the 
tank, and note how many gallons it takes to fill up the tank. And then do the 
obvious math.)

There's even more evidence:

http://www.mpgforspeed.com/

http://www.fueleconomy.gov/feg/driveHabits.jsp

http://editorial.autos.msn.com/article.aspx?cp-documentid=1090496

http://en.wikipedia.org/wiki/File:Fuel_economy_vs_speed_1997.png

Hey, I'd rather it be different, but it isn't. With highway driving there is a 
trade off between speed and mileage. Time saved vs. gas saved. You have to 
drive with the laws of physics you have, not the laws you might want.


AEF