Hey guys. Added 35x12.5s to my 2016 eco boost after having 275/65/20 and used a scanner to change the tire size in the computer as it was still running the stock 275/55 size. Selected 315/70/17 as that is the same 34.5" diameter as my 35" Toyo at3s. Speedo is spot on but my shift points seem a bit off especially under mild acceleration leaving second gear, just wants to hold second for what seems way longer than before. I don't believe it was like this with the 35s before changing the speedo. Any suggestions? Also feels somewhat normal in sport mode.

 

Check the speedo (use the engineer/test mode so you see the true speed, not what the needle shows) against GPS to see if you're actually calibrated. The computer is programmed in revs/mile, not diameter, so you may still be off.

Did you reset the adaptive memory after the change? If you haven't, you should.

 

Speedo is spot on based off of my gps radar detector. I did not reset the adaptive memory. I will look into doing that.

Part of me thinks maybe this is how it's supposed to be as when I had the previous tires the speedo was still off. Just doesn't feel like it should hold it for so long.

 

The larger tires are causing a greater load for any given speed so shifts may be delayed a bit. Similar to running with a tailwind vs headwind it can hold a gear longer or shift sooner based on when it feels is a good time to do it. I know on my truck it'll shift to 6th if I bump the cruise control up to 49mph from 48 on level ground and low wind. On a windy day it may hold 5th on the trip up but go to 6th quicker on the trip back.

You could also have it be doing some relearning and it'll smooth out over time.