Tuning also plays a part in this role.....

 

At relatively high speeds like 65mph wind resistance plays a major role on per car basis. So it may be on average a good speed. The easiest way to figure out what speed is best is to balance the speed with the rpms. The lower the rpms the less gas consumed, and the higher the speed the more distance traveled. So when your going at 5mph in first, 20mph in second, 30ish in third, etc you should be arounf 1,500-2,000 rpms. So essentially you are consuming more or less the same amount of gas, a little less the faster you go because of the friction, and wind resistance. So if your in 5th gear, and you maintain it around 1,7000 not too much over 2,000 I figure your doing optimal.

I will start testing this theory when I start going to a new university across the state, that is 100 miles of interstate driving. I will get the MPG at different RPMs at different runs across. That will be in 2months so it will take a while.

 

I found in my car that if you stay between 2300 and 2700 RPM's(2500 is the sweet spot) I usually get around 32-38 mpg, depending on wind grade and load