Ok folks need some help. Took my truck in for service, also asked them to take a look at teh rear end, as it has been getting a pretty loud whine/ roar. As I expected the axle bearings are shot. They quoted 1400 to rebuild. Talked to teh folks in Raleigh at East Coast Gear Supply ( great prices and reputation) They will swap both axles to 4.11s and rebulid the rear end for a couple hundred more. Whet I really need to know is wha did everyone that made the switch to 4.11s,mileage do? I am currently running 3.55s, with 33" tires, and averaging around 13.5 mixxed driving. I would be real interested to hear how everyones mileage changed when going from 3.55s to 4.11s. If I am going to lose a ton of mileage it isn't worth it, otherwise it is a no brainer.

 

Sort of depends on what kind of driving you do most often. It would be a 14%increase in rpms to cruise at the same speed as it would with a ratio 3.55. That said if you do alot of city driving than the engine isnt working as hard to move the vehicle w/ 4.11. 2mpg loss would be a reasonable estimate IMO. It will certainly feel better though with those 33s. You sure its not just the pinion bearings wining? Both axles for 1400 aint too bad.

 

Not sure which bearings. I have to pick it up this evening, and talk to the tech. My first thjough was pinion bearings, I will know for sure this afternoon.

 

Your city mileage might increase. You shouldn't take much of a mileage hit on the highway unless you normally drive considerably over the speed limit.

 

My mileage increased an average of 1-2 mpg across the board! 3.55's to 4.10's rolling on 33's!

 

I usually run pretty close to the speed limit. Of course a lot of that is 70 mph. According to my calculations including squish, Rpms should go up about 11%.

 

You will be turning some higher revs on the highway, but it is going to be offset by better efficiency and drivability. I'd do it and not look back. I had a similar set up in '92 Bronco.

You're cruise rpm will increase by about 15% at any given speed. So it you're turning 1,900 rpm at 70, you'll be at ~2,200. That's not that significant of an increase in rpm.

 

Keep in mind, rpm's is half the equation. Engine load is the other, and the one that will be quite reduced. That's where the fuel economy increase comes from.