Speedometer correction for larger tires

[Jimmy07]

Ive used Alfa obd ,tazer DT, and Diablo sports trinity 2 EX (not all together)
I can’t remember the exact size I entered on Alfa,I’ve since then reverted ecu back to stock tires because the code would bug me and I just allow for the difference in speed in my head now. But I googled the metric size of 295/65R20 and tazer and the trinity 2 allow you to enter the tire circumference in inches which I measured with a tape and entered.
The only way for me to clear the line mismatch code was to reset to a smaller tire.
And yes I have park assist.

And I’m not the only person who has ever had this happen. A simple google search will pull many other people with same issue. The jeep guys also report losing park assist. But that never happened to me. But it was annoying seeing that code every time I scanned my truck. So I learned to deal with the Speedo being off by 5 mph at 55 and above.

That is a known issue with the park assist module reporting that, hence why park assist gets disabled. It will happen if you adjust the tire size to over somewhere around 2700mm. You either have to stay under that number, deal with no park assist if it’s over, or use the Z automotive park sense fix cable.

 

[Alexrm250]

Is there any new info on speedo calibration affecting transmission timing on gear shifts? I feel like I'm reading conflicting information from past threads. Thanks for the help.

 

[Ready Go]

Is there any new info on speedo calibration affecting transmission timing on gear shifts? I feel like I'm reading conflicting information from past threads. Thanks for the help.

I don't know that I've heard any conflicting info here..if you change your tire size and don't adjust your speedometer calibration, your trans shift points will be off..and not in a good way, it'll be sluggish.
Use AlfaOBD or Jscan for adjustments.

 

[Alexrm250]

I don't know that I've heard any conflicting info here..if you change your tire size and don't adjust your speedometer calibration, your trans shift points will be off..and not in a good way, it'll be sluggish.
Use AlfaOBD or Jscan for adjustments.

Ok thank you. I have a 23' rebel with the digital cluster and park sense. Bought it used with 285/75/R18s. I plan to recalibrate using AlphaOBD but really don't like the idea of losing my sensors. Any experience with the Ram Parksense fix cable from Z automotive? I understand having transmission problems is worst than having some sensors but not working but looking for a clean way to do this. Thanks again.

 

[Ready Go]

Ok thank you. I have a 23' rebel with the digital cluster and park sense. Bought it used with 285/75/R18s. I plan to recalibrate using AlphaOBD but really don't like the idea of losing my sensors. Any experience with the Ram Parksense fix cable from Z automotive? I understand having transmission problems is worst than having some sensors but not working but looking for a clean way to do this. Thanks again.

Technically, your 285/70R18 measures to 2779mm, so over that magical 2700mm limit that causes the sensor disable, but here's a caveat to that...if you run lower tire psi, the OAD is smaller and that helps.
I think I recall Rebels run 55F/45R tire psi(?). You'd be more comfortable running lower psi (unless towing, then keep OEM suggested psi).
I run a 285/60R20 which is 33.5" tall and 2670mm circumference, but I don't run the OEM Laramie 39F/39R tire psi, I'm running 38F/34R currently (which is considerably more comfortable) and my tires aren't lifting (or as tall) as much as expected and I only had to adjust my AlfaOBD programming to 2570mm (which AlfaOBD propagates to a 275/60R20 tire) to get my speedometer spot on and GPS verified.
It seems strange, but does make some sense..the 285/60R20 should be 33.5" tall, but really only lifted the truck height as though it were a 32"..which a 275/60R20 is (but that's a generic rule, not all tires match the rule, and my Dueler AT Ascents seem a bit short..especially with lower tire pressures). BFGs are known to be short on the calculated scale.
Long story short, plug in a "mm" number just under the 2700mm magical number, and maybe lower your tire psi, and pray for no disable.
I'm planning to step my 285/60R20s up to 285/60R20s at some future date (my tires only have 3000miles on them). Those measure 34.8" tall on the calculated scale like your 285/75R18s, so I'll be in that same boat with you.

 

[Alexrm250]

That does make sense. I appreciate the feedback. I'll give it shot with the PSI. Currently running ~55psi all around on Nitto Ridge Grapplers.

 

[Ready Go]

That does make sense. I appreciate the feedback. I'll give it shot with the PSI. Currently running ~55psi all around on Nitto Ridge Grapplers.

These DTs are lighter in back, so seem to ride better with lower rear tire psi. Carli suggests 35F/30R, the lowest I've tried is 36F/33R and I liked it, but it's hard to regulate that lower psi in Wisconsin winters when we go from 32° to 0° to minus° swings.
Recently when attending a late night Xmas party with my truck sitting parked in 6" snow at 0° for hours, my rear psi dropped to 30psi and trigger the Tire PSI Warning & Engine lights. After filling up to 37F/34R, both lights went back off after a quarter mile, but I did go into AlfaOBD and reset the "Adjust Tire PSI Threshold" to 36F/33R (although in AlfaOBD it shows 35F/32R..it redid it a number of times and each time it's the same).
The tire psi can be way high over the set parameters without a problem, it's the low readings that trip the warning lights.