Originally posted by hollaboyz
doesn't it work the sam way as gearing down? you gear down, you dont waste more gas. i cant see being in 5th giving you that much better of fuel economy. if you floor it in 5th, aren't you opening up your injectors and injecting way more gas into your engine? im no expert though 
h:
You use more gas in lower gears because the engine turns faster than the gears in the transmission. This is called gear reduction. In lower gears, you need more torque to make your car move. In an overdrive gear (generally 5th gear), the engine turns slower than the gear in the transmission so you are using less gas. You don't need much torque anymore because your car is moving fast.
Hopefully that makes sense to you.