Just wanted to wake up this thread to see if anyone has any new thoughts/Experience on changing gears vs. Gear Vendors OD... I am interested in the Detroit locker 3.90's but the 4.30's would be fine in my application. The 4.88's just aren't practical unless you're trying to pull the devil out of hell...

 

I had an F550 with gear vendors and it did not make a big difference in fuel mileage. I got 11 mpg instead of 10 and it does not shift smooth if you have an auto unless you anticipate and match engine speed. If you shift under full throttle it will bang so hard everything shakes in the cab same thing if you downshift. Also have a gear vendors on an F350 with 4.10s and fuel mileage was barely any different. The only benefit was the in cab noise was less. The lube capacity was tiny and there was no extra cooler for it so towing heavy in OD is a no.

If you are looking to regear an F550, the 4.88 carrier needs to be replaced with the next lower carrier since the carrier that works with the 4.88 R&P does not work with a 3.73. These are hard to find by the way.

If you are looking to regear an F450 and it has the Dana 80 (Ford did not add the S14-110 to the F450 until later 2003 as far as I am aware), these are not a drop out design like the S135 or S14-110 rather they can be serviced through the rear diff cover and changed gearing like a regular light duty rear end. Gearing changes are less involved since the ring gear is not riveted to the carrier.