Your driver's door information panel should have your F150's axle code which will tell you what gear ratio and differential your truck has stock. Figuring out whether you have a 9.75" or 8.8" rear end will require you to look at your differential. The 9.75" has 12 cover bolts and is an oblong shape with the passenger's side narrower than the driver's. The 8.8" has 10 cover bolts and a fairly symmetrical overall look. If you're still unsure, call us directly at 623-434-5277.