The single most $$$ saving step i have taken individualy has been to slow down. After doing some tests of my own and having some friends try some as well, by simply driving at the posted speed limit i have been able to jump my mileage back and forth to work (~70/30 highway/city) from 21.5 MPG to 27.5 MPG in my 2001 Pontiac Montana van (V6)
here is what i did:
1. stopped my typical jack rabbit starts/stops
2. started locking my cruise control at the posted speed limit (most of my drive is 55MPH)
By merely doing these two things, i was able to get another almost 28% out of my mileage. it is difficult at first to watch cars zip by in the left lane, but after a day or two i found the drive more enjoyable, stress free and of my 22 mile trip one way all i added was 2 minutes to my comute. i used to drive 5-10 MPH over the limit with other traffic in the left lane on the highway. i tested my new tequnique with a co-worker who typically drives 10-15 over. we met at a starting point about 20 miles from work and i drove the limit and he drove his typical speed and i pulled into work 1 minute and 40 seconds after him. Two things impact this
1-over short distances speeding buys you very very little time (time = distance/velocity do the math yourself 20 miles at 55mph = .36 hours = 21.8 minutes whereas 20 miles at 65 mph = .31 hours = 18.5 minutes)
2 - if you have even just a few stop lights, your drive time will be greatly impacted by them. on my 22 mile trip i have 4 lights that i may hit. one thing i remember when i used to speed was that i would pass people on the highway only to have them catch up to me on the offramp stoplight - who looks foolish then?
As a Mechanical Engineer i need to see the math behind this sort of thing and it is easily explained in the aerodynamic drag equation
Drag = .5 * Coefficient of drag * density of air * Velocity^2 * Area of shape breaking the wind.
you don't need to know a lot about math or algebra to understand that the velocity term in the equation is squared meaning that the faster you go the more drag you have to fight exponentialy! this is why freefalling objects reach a terminal velocity, the drag force equals the gravity force and their acceleration stops and they maintain the same freefall speed. the same thing happens in your car, the faster you go the more and more power your engine has to provide to over come this drag force. it doesn't take much power to keep your wheels spinning to keep you at say 55mph, especially on flat ground, most of your power goes into breaking the wind resistance, aka drag.
try it for yourself! my friend did a little experiment driving 60 miles at 65mph on the highway after a fillup and filled up after the 60 mile and got 47mpg out of his new civic coupe (no AC, no extra stuff in his car, windows up!)