Driving 55 mph does save gas mileage
The old rule that driving no faster than 55 mph to save on gas still works. This guy tested a Chevy Malibu, and got an average of 35 mpg. However when he went bit over 70 mph, the mileage dropped sharply -- to 25 mpg.
For every mile per hour faster than 55 mph, fuel economy drops by about 1 percent, the drop-off increases at a greater rate after 65 mph. The faster you go, the faster the fuel goes.
There are costs the slower you go, however. It took 49 minutes longer to make the trip at 55 mph -- three hours and 36 minutes total -- but it seemed like forever. Sitting in the slow lane on the highway, tapping the gas pedal to maintain a steady speed of 55 makes the car felt like it was traveling at 25. Click here to read all about the 55 mph test.
*UPDATE* Only about 15% of the energy from the fuel you put in your tank gets used to move your car down the road or run useful accessories, such as air conditioning. The rest of the energy is lost to engine and driveline inefficiencies and idling. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous. Read more here.
*UPDATE* I came across a tip that claims if you wash and wax your car, you can improve your gas mileage by up to 7%, which based on today's prices would be about $.21 per gallon. Not too shabby.The basic idea behind it is, after washing and waxing your car, the surface of the vehicle is much more likely to be nice and smooth, therefore, it should be more aerodynamic than when you have lots of bug guts smeared all over.
It looks like a sound theory, but I don't know if simply cleaning your car will increase your fuel economy that drastically. At the very least, your car's going to look nice? Source.