The Great Compromise

new 9/1/02

We all enjoy driving our SHOs, and we all enjoy driving them, if we can be tactful, with a "sense of purpose." In general we know the faster you go the worse gas mileage becomes. It might be worth trying to quantify the relationship as gas prices increase.

Blessed with an AutoTap I took a drive on level IL interstate and kept incrementing the autopilot in 5-mph increments recording air flow at the MAF, engine rpm and mph. I threw out the high and low values as the SHO adjusted to new speeds. This left me about 140 MAF readings for each 5-mph increment.

Lets see where this can take us. We can directly read MPH and MAF air flow rate. Because of the tendency for the actual mph to vary slightly (and the potential liabilities of spending extensive time at high speed) the last two velocities were less precise and have less data.

 

MPH	MAF Air Flow Rate (lb/min)	lb per hr fuel	Gal/hr	MPG
40	1.7757	7.2447	1.2074	33.1277
45	1.7820	7.2706	1.2117	37.1361
50	2.5741	10.5023	1.7504	28.5651
55	2.7583	11.2539	1.8756	29.3233
60	2.7721	11.3100	1.8850	31.8302
65	3.4436	14.0499	2.3416	27.7582
70	3.9646	16.1754	2.6959	25.9653
75	4.6620	19.0208	3.1701	23.6583
80	5.0498	20.6033	3.4339	23.2972
85	5.8902	24.0320	4.0053	21.2217
90	6.7351	27.4791	4.5798	19.6513
96	7.8202	31.9065	5.3177	18.0528
100.5	8.3004	33.8655	5.6443	17.8057

14.7 A/F ratio is .068 F/A ratio; so for example 1.7757 lbs/min air requires:

a gallon of gas weighs about 6 lbs

7.2447 lbs/hr =1.20745 Gal/hr

and of course mph/gph gives us mpg

That is best steady state case, no fuel used for acceleration, level grade under ideal conditions. You may never get that but it is a repeatable number. So now we can plot mpg against mph.

The curve looks smooth above 65 mph, below that the field data rolls about and I have to wonder if the odd results are a function of converter lock-up, coolant temp, or ignition timing which are more important than aerodynamic drag at lower speeds. We can for simplicity fit a 3^rd order polynomial curve that can help us extract a few general rules of thumb. Using the trend line the change is about 1.5 mpg/5 mph. That may give you something to thing about as you set the autopilot. If you don't tell your AWA we will not either.

Based on $1.50 / gallon you can see a calculated cost (excluding citations) for a 600 mile trip on the next chart. The speed limit is 65 in my neck of the woods, I have to drive 80 mph to average 75 mph. Using the table I spend ($39.65-$32.49) seven bucks more but arrive 100 min. sooner. Before we all turn into Ralph Nader on some days (again tickets excluded) that may be a good deal. My time is worth at least $4.00 per hour.

You will never get this kind of gas mileage in traffic, or in mountains, or with a car full of beefy co-workers. The wind was calm with gust up to 5 mph the night I did this and the roads were dry. I have gotten close to 23 mpg at 75 mph and those with intact exhaust systems may actually do better!

We might speculate what the gas mileage may be above 100 mph. It could look something like this. Who knows? The table above spans 1,470- 3,570 engine rpm. All rules would change as the secondaries open at 4,000 rpm and 140 mph relates to 5,000 rpm which is peak volumetric efficiency but which is offset by the aerodynamic drag that becomes huge above 100 mph.

I can think of worse risks above 100 mph than poor gas mileage. I’ll leave that field research to another soul.