Swapped 17" Wheels and Tires for 20's. Final Drive Question.

[TMyers]

I swapped my stock wheels and tires from 265/70/17's to 285/60/20's. My truck (Cummins) came with the standard 3.42 geared rear end. The 265/70/17's were 31.6" tall where as the 285/60/20's are 33.1". This changes the "Final Drive" gear ratio. The speedometer change is not something that concerns me. At 60 MPH from mile marker to mile marker, I'm 3 MPH fast.

I know it's most likely in my head but my "Take-offs" feel a little sluggish now. I have always eased into my take-offs but if I spool the turbo a little, it's like a rocket off the line. It may just be my driving habits need to change.

My question(s) are: Do CTD Rams, with 20" wheels, have the 3.42 rear end as standard equipment? If so, are the transmission shift points changed for the taller tires/wheels?

 

[Scottly]

275/70/18 is a factory option that is on my 2018. I believe you can have the dealer load in that option to your ECM/BCM, and the speedo and shift points will correct themselves. I may be wrong.

 

[Big Terry]

Taller tires mean less revs to cover the same distance, so your engine isnt going to rev as high to get there. that is what will cause your 'sluggish' feeling.

not totally sure about how common the 3.42s are but gut feeling is they are more of the norm than say, 3.73 or 4.10s, especially in SRW configuration. Shift points could be slightly different depending on the rear end gearing but I would assume that different (stock) tire sizes wouldnt necessitate a tranny reprogram, not from the factory at least. If it makes that much of a difference to you, look into getting a tune done on your trans. then youll be able to tweak it however you feel it serves you best.