marcdeluca
12-09-2004, 07:11 PM
I have the Snap-On MT-2500 scanner. I found that it will report microliters of fuel injected per injection. So I thought to myself, "Self, I wonder if I could calculate the instantaneous fuel mileage from this data?" At 60 mph, the scanner reports 1820 rpms, which means the engine turns that many rpms in a mile. This could also be found by dividing the feet in one mile by the tire circumference, which comes to 687 tire revolutions/mile with a 245/75/16. 687 times the rear end ratio of 3.73 comes to 2563 driveshaft revs/mile. That, times the 5th gear ratio of .71 equals 1820 revs/mile. A 4 stroke 8 cyl engine fires 4 times/revolution, so 4 times 1820 is 7280 injections per mile. At 70 mph on flat road, the scanner reports 1 uL pilot injection and 25 uL of main injection, for a total of 26 uL. 26uL equals .000026 liters/injection. This, times 7280 injections/mile equals .189 liters/mile. Divide this by 3.785 liters/gallon equals .050 gallons/mile. Since mileage is usually figured by miles/gallon, take the reciprocal of .050 to get gals/mile. This is 20 mpg. To save doing all of these conversions everytime you figure a different uL injection size, I came up with the magic number of 520. This is a constant as long as this tire size, rear axle ratio, and transmission ratio is used. For example, if I see a constant 35uL injection size, I take 520 and divide it by 35uL, which is 14.9 mpg. When I have my propane on, the injection size drops to 18, which comes to 28.9 mpg. If you are towing in 4th, the constant would change to 369. As you can see, I have way too much time on my hands while driving to have done this little exercise. I think I need a nap now.