Hey guys, hope you can help me out here. I purchased a 08 mega cab with the 6.7....Lifted it and put 38" tall tires on it.

Understand gas milage is bad, but sometimes the tranny has issues shifting, like it cant determine which gear to go into, seems maybe it just goes back and fourth into next gear and previous gear really quick so it jerks the truck around sometimes.

Assuming that its because of the large tires but whats is the best way to handle this. Im sure I cant be the only one who has put large tires on the new 08's. If I need to change my gears, what should I go to? I have 373 right now, aftermarket intake, and edge /juice module

,, not sure that 410 will be high enough... 456??? Should I go to the dealer to have them reprogram the computer? Will they make notes and possibly have issues of warranty in the future? Thanks for any input...

 

I think the shifting back and forth is because the computer is reading the speed correctly. With today's electronics everything needs to be right for the trannies to shift right and the abs to work correctly. Your gearing may be a little high though. With 38s I would probably look into some 4.56s.

 

Yes you should change gears to match the 38" tires and what warranty?? I'm sure with 38" tires most of the drivetrain would be voided!

 

I have 4.10s in my truck from a chuby with 37's and reprogramed the computer to match the tires.......truck drives great at 1900 rpm at 70mph even increased my mpg by 1.5 miles per gallon A buddy of mine has 37's and he put 4.56 gears in his truck, but he is at 2150rpm at 70mph and his gas mpg got worse.....I think diesels act differently than gas motors as far as gear changes are concerned.
Thats my $.02

 

Thats what I thought, really appreciate the thoughts. But with 38's maybe I can go to 456? and still be ok? Did you have the 6.7 with the 6 speed auto? Here are some pics of what I did to the truck.

 

no don't have the 6.7 with the 6 speed auto.....maybe with the extra overdrive in the new autos, the 4.56's will even out pretty good.

 

Sweet. I like that. Go with the 4.56 and the 38's. If you think you went too low (numerically high) with the gears then just go a bit bigger on the tires...

 

i have 37's on my truck with 3.73's and the 6 spd auto..... i had the dealer calibrate the tire size the highest that they could and the speedo is right on the the tranny shifts just like it should...no gear swaps needed

 

Maybe I should just go to the dealer first and see what they can do first before spending the money on the gears... I did call the shop that would change the gears and they said you wish you would of done it sooner,, will save me in gas milage. I probably agree with that but will try the dealer first.