no ruler. use a measuring tape. the length (from the driver side wall to your passenger side wall), the heighth (top to bottom), and depth (back of your truck to front). just remember to write down the MAX dimensions you are willing to work with. if your overall length is 40 inches, but you can only use or want to use 30 of those inches, then write 30 since that's what we have to work with. being a single cab...you'll have a lot of length, maybe about half that for the heighth, and not so much depth. but we'll see what we can work out