Alternative MPG Calculation

[aerohead]

Originally Posted by XCRN

I just wanted to verify my thinking and math with the tools I have. I run a Holley Terminator X and I can take logs of short trips and sections of drives. In the logs I can see my overall fuel flow in lb/hr and speed in MPH. I setup a spread sheet of a log of my daily commute to work and the average speed was 35.87 mph and found the average fuel flow to be 12.75 lb/hr for the whole trip. My question is the formula based on these values. I have been using this formula.

(speed)/(fuel flow)*(weight of gas/gallon)

My understanding is a gallon of gas is 6.3lbs. In the above case I got 17.44mpg but at the pump doing the tradition calculation I have been multiplying the miles by 1.09. I have comparred the Jeep ODO to a GPS and the GPS reads higher by about 9%. So when I multiply the speed by 1.09 and do the calculation again, I get just over 19mpg.

Does my barely passed highschool math check out or am I wrong? I have been making some lean tunes and so far the best I have gotten was 16mpg for the last 2 tanks using a corrected speed and 13mpg before that, so I am a bit skeptical I did this good.

E10 Regular Unleaded is 6.138-pounds/gallon, if applicable.
I have a GARMIN NUVI and it agrees , within whole mph, of the Southern California Timing Association's equipment at Bonneville.
Gasoline has a coefficient of thermal expansion, so technically, it's 'weight' per cubic volume will vary slightly throughout the year ( Pilots use 'pounds per hour' rather than miles per gallon).
If you vary your driving speed on your commute, it's likely that your engine's brake specific fuel consumption ( BSFC ) is also moving around while you drive.
In a best-case-scenario, you'd accelerate to some speed, and just hold that speed for the duration of the test, otherwise you introduce so many variables that it's virtually impossible to make later sense of the data.
If you have an official measured-mile along a nearby highway, you could check calibrations of both the odo and gps.