First you need to know your miles per gallon. You can do this by attaching the trailer once you've filled the tank (write down the current mileage) and drive it for 30-50 miles. Fill it up again and figure out how many miles you've gone and divide that by how many gallons you added.
Or you can find the gross vehicle weight, on a sticker on the inside of the driver's door and estimate what the mileage might be with the added weight. Good luck.
〉 Answered on Jun 4th, 2008 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines