Originally Posted by budford

I guess I have one of the odd F-100's since mine has the F-150 class brakes. It's a 4700 lb rated short bed built in Dec. of 1980.

Originally Posted by 1986F150six

For the original poster... if you change from the original 2.47 to a 3.55 differential, that is a 43.72% change which is huge [ask me how I know]. Your speedometer will indicate 79 mph @ a true 55 mph.

My son's truck with like change is definitely much more fun to drive and lost about 2-4 mpg on the interstate.

Originally Posted by kedwinh

That's exactly what was done with mine. Don't know the MPG change but, it is easier to drive. I've thought that a 3.08 would be a good compromise if towing wasn't important.

Originally Posted by kedwinh

That is a cover, like a dana44 or 10 bolt, just different shape. They bolt onto the rear of the diff just like the others do. Think the Ford 9" is the only one with a removable pumpkin. Most all other diffs have a axle mounted pinion/gear/axles and a removable cover to service/change fluid. Unless I'm not understanding what your saying, which is REAL possible.

But either way, he's got some work to do if he wants to safely pull anything. I helped my friend swap rear ends on mine before I bought it from him. It had the same setup I think this guy has and it would hardly get out of it's own way empty. SROD 4sp and 2.47 gear even with the factory 215/75 tires is geared way high.