I have a 2016 ram express. I upgraded my tires to 35" from the 33s that were stock. I have a Matco MaxMe tablet to change the tire size for the speedometer. I see that the computer requires the figure in mm for circumference. I believe the 35" tires are 2791mm circumference. However, when I set the computer to that size, my speed is still off. When I set the cruise to 60, my GPS says I'm doing 57. I tried going back to the stock size setting, set the cruise to 60, my GPS speed is 62-63. I can't find any other setting that the computer will accept. My steering would also not work properly, studdering and stiff. I get 4 wheel drive temp not avail. I don't know how to reset the BCM or TCM. I have DTC codes (00,03,d4,12), P1614-00. I was also getting P1DB6-00, P215C-00. I think the only one now it P1614. My check engine light was on for a while. I got it turned off but still have the 1614 code. Sometimes the light would come back on after a restart. I did get it to stay off.

 

UPDATE... Well I found that I was missing a step in this process. I did not go back through and initialize the ABS ECU after I made the changes. I went to the company that I bought the tires from, they said the tires are 34.8" dia. So I went back in the BCM, made the changes. Went back to the ABS and re-initialize. Test drive... speedo still off. Went back to tire size, I ended up setting the computer to 33.5" and my speedo is NOW correct. I don't understand why I had to go to 33.5 not 34.8. Doesn't make sense. Next question... I had also changed my gears from stock 3.21s to 3.55s. It's my understanding that rear gears DO NOT effect the ECM in judging speed. My Matco MaxMe doesn't even give me an option to adjust gearing. I know the HPtuners will. Not sure if that will be my final correction. I will post back after my drive to work tomorrow to see how it's shifting. That is my next test. So far it seems proper...

 

Nope. The speed sensor(s) are after the gears, so, they simply don't matter.

It *may* confuse the PCM though, if it expects a certain RPM at a certain speed..... but, if you can't tell the PCM would the gear ratio is, I am going to guess that it won't matter. Drive it for a while, and see how it goes.

The tires may be 34.8" diameter just layin' there on the shelf, but, mount it to a rim, and rim width will affect diameter a little bit. Setting the weight of the truck on it, will affect it quite a bit.

 

I know in the older cars, before the big computer and canbus interface, it was all done through the output of the trans. I know that it is now done at the wheel ends. I was back in the BCM again today and looked through the current config. I did find where the computer shows the current axle ratio, mine says 0. So it's not even showing any gears. Now, I"m not sure if that is the major player with the issues or not. Something that I have to look deeper into. My Matco MaxMe won't let me make any of those changes. So now I'm reviewing different tuners, something that I can get deeper into making changes.

 

Wonder if that means the BCM simply doesn't care, or, if it is a parameter the scan tool doesn't know how to interpret..... Either is a possibility.

 

Makes ya wonder