Today's streamlined cars cut drag like never before, but it takes more than driving a jelly bean to break the wind. Air resistance is also a function of speed; the faster you go, the more air piles up in front of you and the harder your car has to work to shove it aside.
The same physics that once made the sound barrier seem insurmountable to aircraft are costing you 15 percent more fuel for every 10 miles per hour (16 kph) you push above 50 (81 kph) -- possibly more, if you're an SUV with a big V-8. Do the math: For every thousand miles driven at 60 mph (97 kph) in a 25-mpg car, that translates to 6 gallons (nearly 23 liters) more bought at the pump. Given the June 2013 national average of $3.63 per gallon, that's $21.78 less in your bank account [sources: Car Talk; DOE; EEA; EIA; GSA; Reed].
And there's another reason to seek life outside of the passing gear: engine efficiency. When driving a manual transmission vehicle, learn to shift efficiently and get your car into high gear as soon as possible. You want to keep those rpms low -- the slower the engine turns, the less fuel you burn. If you drive an automatic, you can achieve a similar mileage boost by using the overdrive gear [sources: Car Talk; Yetiv].