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.
|
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 |
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 3rd 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!
|
MPH |
MAF Air Flow Rate (lb/min) |
MPG |
trend MPG |
change mpg for each 5 mph |
600 mile trip - $1.50 gal |
Hrs. 600 mile trip |
|
40 |
1.7757 |
33.1277 |
35 |
$ 25.71 |
15.0 |
|
|
45 |
1.7820 |
37.1361 |
34 |
1 |
$ 26.47 |
13.3 |
|
50 |
2.5741 |
28.5651 |
32.7 |
1.3 |
$ 27.52 |
12.0 |
|
55 |
2.7583 |
29.3233 |
31.1 |
1.6 |
$ 28.94 |
10.9 |
|
60 |
2.7721 |
31.8302 |
29.5 |
1.6 |
$ 30.51 |
10.0 |
|
65 |
3.4436 |
27.7582 |
27.7 |
1.8 |
$ 32.49 |
9.2 |
|
70 |
3.9646 |
25.9653 |
25.9 |
1.8 |
$ 34.75 |
8.6 |
|
75 |
4.6620 |
23.6583 |
24.2 |
1.7 |
$ 37.19 |
8.0 |
|
80 |
5.0498 |
23.2972 |
22.7 |
1.5 |
$ 39.65 |
7.5 |
|
85 |
5.8902 |
21.2217 |
21.2 |
1.5 |
$ 42.45 |
7.1 |
|
90 |
6.7351 |
19.6513 |
19.7 |
1.5 |
$ 45.69 |
6.7 |
|
96 |
7.8202 |
18.0528 |
18.4 |
1.3 |
$ 48.91 |
6.3 |
|
100.5 |
8.3004 |
17.8057 |
17.2 |
1.2 |
$ 52.33 |
6.0 |
|
Speculative |
||
|
MPH |
Change |
MPG |
|
105 |
1.2 |
16.0 |
|
110 |
1.2 |
14.8 |
|
115 |
1.1 |
13.7 |
|
120 |
1.1 |
12.6 |
|
125 |
1.1 |
11.5 |
|
130 |
1.0 |
10.5 |
|
135 |
1.0 |
9.5 |
|
140 |
1.0 |
8.5 |