Fuel efficiencies

noted by James Hamilton:

Air resistance increases the faster you travel, which might lead you to think that higher speeds always require more fuel. However, your car's engine is designed for maximal efficiency in converting fuel into motion when you drive at higher speeds. As a result, the typical car gets much better gas mileage if you drive it at 45 mph instead of 15. However, at speeds above 60 mph, the wind resistance becomes a dominant factor, and miles per gallon for most cars starts to decline significantly if you drive faster than 60.