My first posting - maybe this is the best place for it??
The following site will help you estimate gas mileage for a given vehicle under various conditions. Put in the known values, then fine tune the result by changing "Engine Brake Specific Fuel Consumption" and "Drivetrain Horsepower Loss" values until you get a mpg figure that you know is close to reality. Then, change only the "Vehicle Miles Per Hour" or "Vehicle Weight" or "Tire Inflation Pressure" to see how those changes affect fuel mileage. Its a good way to get an idea of how speed, weight and/or tire pressure affect mileage. Here's the link: http://www.bgsoflex.com/mpg.html
Its best if you can enter the coefficient of drag, and a web site where you MIGHT find the coefficient of drag for your vehicle is: http://www.carbuytip.com/car-specs/09-c ... et/lumina/
I say "might" because it gives the info for most vehicles, but not for all.