"Find the miles per gallon your car gets on the highway "

Basic math. Fill up the car and write down the miles on the odometer. When you fill it up again write down the gallons and the miles. Subtract the first miles from the second which will give you the miles traveled. Divide the miles by the gallons and you will have the miles per gallon. The fuel economy will vary with the engine you have, the gear ratio and how you drive.