So my brother and I have both just recently purchased F550's that are equipped with 4.88 gears and 6 speed manual transmissions. A 2006 and a 2010 both diesels. We spend a lot of miles with them traveling down the road to jobs and the 4.88 gears are way too low for highway driving. We both plan on doing something too change this. I think we want to get to about the equivalent of 3.73's as our interstate speed is 80. I'm not even sure 3.73's exist for the Dana super 60. Maybe an auxiliary gear box is something to look into. I figure someone on here knows more about this stuff than me.

 

Check Yukon to see if they have a gear.

https://www.yukongear.com/difflist.a...&DriveType=RWD

 

I've got a 550 DRW Auto with GearVendors Over/Underdrive. I've only put 600 miles on it but with 4.88's in the rear I pull 2000 RPM. I don't what it's like without it just thought I'd throw it out there..

 

It looks like a Spicer 5831 "Brownie box" might be a good option but I don't know much about them. I think this could be an option to have 2 overdrive gears. Anybody have or know of any good resources on these?

 

The best source of information I know about is a great thread on auxiliary transmissions here on Ford Truck Enthusiasts: https://www.ford-trucks.com/forums/5...ie-thread.html