I have a manual and it came with the 265/70 tires. The door code says b07a. I got conflicting info from posts on different boards that said I had 4.30's, 4.10's or 3.91's. I finally jacked up the rear, spun the tire and counted the driveshaft rotations and came up with 4.30's. I repeated the test a couple times to verify. How do you know you have the 3.91's? What rpms are you at traveling at 70 mph?