Tire pressure matters
Driving on under-inflated tires is like walking in sand: the resistance forces you to work a lot harder than you would otherwise, sapping your energy. Check the pressure in your tires every time you get gas, and add air even if they're just a little low. A sticker on the inside of the driver's door will tell you the manufacturer's recommended tire pressure. Don't wait until your vehicle's tire pressure monitoring system kicks in, since it'll only warn you after a twenty percent drop in pressure, which indicates severely under-inflated tires.