Every time I fill up I write down (and reset) the miles since my last filling, the number of gallons purchased, and information from the car's trip computer (average mpg, average speed, and gallons consumed). I then plunk the data into a spreadsheet.
We have an '02 Chevy Impala (with the 3.8 liter V6 IIRC). Today, we filled up and crunching the numbers shows we had a mileage of 30.31 miles/gallon over the last tank. Not bad, we have been averaging 24.85 since early April of 2007.
What is the difference? Maintenance? A magnet on the gas line? One of those pills that turns water into fuel? New brainboxes?
Nope none of that. Thoughtful driving and sticking to speed limits.
The energy of a moving object is related to the square of the object's speed. That is, if tow identical objects are traveling at 2 mph and 4 mph the energy of the faster object is not 2x more but 4x more. Another factor working against higher speeds is wind resistance is generally proportional to the object's speed. That is (the faster object in our prior example will only experience a resistance force of 2x greater than the first one) the faster you go the more energy you have to pump into the object to keep the speed up.
For example, in order to go from 65 mph to 75 mph you must pump in 1/3 more energy than it took you to get from 0 mph to 65 mph and then you have to expend more power (something like 15% more) to keep the speed at 75 mph. That extra power required needs gasoline.
Of course, one may say "Why don't you drive 45 mph then?", well life is full of tradeoffs. On my trips to/from Madison the difference in speeds results in a trip time of about 14 minutes provided. However, it seems on the way to Madison the speed matters little I usually get to the office in a range of five minutes despite my speed. If I drive fast, I hit the beltline at an earlier & busier time, if I drive slower I hit it later and less busy.