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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:

(1.7757 lbs/min) * (.068 F/A) * (60 min/hr) = 7.2447 lbs fuel/hr.

a gallon of gas weighs about 6 lbs

7.2447 lbs/hr =1.20745 Gal/hr

and of course mph/gph gives us mpg

(40 mph)/(1.2074 gph) = 33.128 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 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.

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

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.

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

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

 

 


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