Gear Ratio

Yep, may want to go 3:40 ish or around there... Maybe 3:50 ish... There are calculators out there to figure it out. RPM vs Gear Ratio, that would be my first quest. I would call my local Cat Shop and talk with one of the guys there. Your particular motor is set up to a certain RPM for best fuel economy vs speed... You need to know those numbers from the factory.

 

I agree, higher gearing ( lower number) doesn't always guarantee better fuel economy. I drove a Century, 500 Detroit, 13, and 3:55's, and that's about as high as I'd go. 70 mph in 13th was like about 1500 rpms, (I'm convinced that truck would do 3 numbers), but for normal traffic, I found myself in 12th most of the time, and fuel mileage ( about 6.9 to 7.1) actually dropped a bit in 13th.

 

Google search Speed Calculator. There are several of them but you put tire size final ratio of trans, and rear ratio. It will give you the speed and rpm for any set up you want. To go to 3.55's from 3.70's is about 150 rpm. It's not enough to spend all that money on. The difference in 3.36 and 3.42 is about 75 rpm if that. Also if the rears are eaton then 3.36 will be your option. If they are Rockwell's then 3.42 will be your option. Personally I think you will spend a lot of money for to little gain. That's just an opinion

 

wore out is correct, you will gain little with rear ratio. I have a 10 speed with 3.55, it was better for me to go to a 13 or 15 speed on paper then to change rears. If I would have made the change to a 13 as researched and planed back in January; I would have paid out the cost in 16 months ( if memory serves me). These numbers were based on 1.0 mpg +/- increase at 2.30 gal fuel. 24.5 highs
Had everything ready to switch over but decided the 5k cost wasn't in the cards this year; just keep her around 65 now which does help a little, and watch my driving habits.

 

Put taller tiers on it.

 

Look into the Borg Warner turbos. You can buy them out right brand new no core exchange for almost half the price of a CAT. And they are more reliable. I upgraded mine long ago. If you look in here for Turbos, you will see mostly everyone change over to the Borg Warner Turbos for performance. Goodluck....