I had to make a trip up to Chi town the other day from St. Louis, it was 299 miles according to Map quest. I filled up with 15.03 gallons, and when I got there I had about 1/8 of a tank, I prob got 21mpg, a little better then I thought, I averaged 70-75 most of the way, except when it rained.
the mileage from the odometer will not be accurate because of the 3.73s
when i had 3.73s, even though i had the closest speedo gear i could get, the speedometer and odometer were not very close. they read faster and farther than reality
Although you can figure how off the speedo and odo are then compensate for it later... but pull the miless off the ODO, gallons off the pump, and then put in any fluff factors (for the gears, tire diameter, etc)...