Wheel and Tire Swap - Gearing 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?​

 

[SnoKing]

If you have the 68RFE transmission, it is an adaptive transmission and may have to re-learn a bit after the tire change. The Aisin trannies have stand alone computers and are not adaptive.

 

[lpennock]

All newer SRW are 3.42. Yes a taller tire will feel slightly more sluggish. It could help some to have dealer program you to the 20 size as it will help computer determine shift points. Debatable if it is worth the hour of shop time they charge to do it though. You might be able to do it for free if they are doing other work that requires computer programming such as the backup warning recall.

 

[Cummins12V98]

Run Tow Haul it will help keep it in the proper gear.

 

[EDankievitch]

Take off's will be a bit more sluggish. And your shift points are now off a bit. I bought a speedo re calibrator on Amazon from ProComp it allows me to adjust for any tire size I desire. The dealer can only choose from a few pre programed sizes. Not only has it kept me from getting a ticket but it helped my shift points and MPG's.
Just one thing any time you bring your truck in to the dealer you should re set the settings back to factory or if your truck gets re-flashed the programed can get locked out and you'll have to up date it to make it work again. Ask me how I know... It's not a big deal just frustrating. I had the shift interlock re-flash done and the dealer set everything back to stock. First thing I noticed was my speedo was off by 5 mph. I'm running 295-70-18's. I went to re set the speedo and was locked out but the system directed me to a phone number and to a tec that was very helpful in updating my unit so it'd work again.