14 Ram 2500 - adjusting the speedo for 35" tires

So I have Alpha ODB and I made one adjustment trying to get my speedo more accurate. I am running 35x12.50R/17 which replaced the stock L285-60R/20E wheels/tires. My speedo always ran 2 - 4 mph faster after the change (meaning when the speedo said 70 I was actually going 66).

So with the Alpha ODB I was about the change the size, however it asks in mm and I went with 2714 and now I am 1 mph fast on the speedo at speeds less than say 40-45 and about 2 mph fast above that.

I actually think it is off by like 1.5 - 1.75 mph as there is some overlap it seems between the gps and speedo which could just be the gps refresh rate.

I am trying to get a feel for how much to adjust it instead of multiple attempts. I know I need to increase the size I input but am not sure how much and I figured I would post it here and see if anyone has any input. Again, I am 2714 right now, should I go say to 2750 or is that too much?

Thanks in advance.

 

What does the tire manufacture say is the tire circumference? They should have it listed then you can convert if needed.

 

I would expect it to be closer to 2700. Using Toyo specs for Rev/mile has 2700mm for a MT and 2691 for a AT.

 

Also, use the odometer to make adjustments off of. Drive 10 miles and see what the error is between the gps and odometer.

 

#ad

 

Nitto lists the size as 109.0 which equates to 2791mm. I saw somewhere (perhaps here) that the actual number to be used would be slightly less which is why I am asking on here. I am sure i can get it via trail and error but given that it is a little bit of a debacle changing it via alphaODB.

 

Is that diameter or rev/mile converted to rolling diameter? There is a decent difference.

 

The Nitto site lists the circumference as 109.9" and the rev/mil as 577

 

Anyone have any suggesting's on what the "right" number would be to use?

I really do appreciate all the responses, the TDR is awesome and has always been a great help.

 

577 = 35.0” for rolling diameter, which is a rolling circumference of 109.8”. That’s 2788mm, and also quite tall for a 35. Most 35’s roll as a 33” tire.

It’s also odd that your speedo ran fast, as a larger tire should make it run slow.

Speedo error is a percentage off, not a specific mph.

 

Well the truck had 20s on it when new, so I am running a smaller tire which would make it act as it is I would think. Either way, I can adjust it but I need to pick what number i am going to try next.

 

The factory 20’s are a 285/60R20 and spin at 626 rev/mile, meaning they are quite a bit smaller than the 35’s you listed. Rim size doesn’t determine overall size, tire size does.

The speedo should be 7.8% slow, not fast.

 

Interesting, well it definitely isn't slow (the speedo), I have checked it using different GPSs as well as phone apps oh and also some radar systems placed on roads to show you your speed. All of those have been pretty consistent.; the speedo is fast.

 

Thinking about this, I had the wheels and tires installed at a local 4wheelparts place in 2015, I wonder if they didn't "adjust" the speedo and jacked it up

 

I didn't see on the Nitto site any spec for Revs per mile. I did find on tiresize.com specs that list 598 rev/mile for the Ridge Grappler G2 and 602 for the Terra Grappler. So I still think the number you should be using is just under 2700.
rev/mile=mm
598=2691
602=2673

 

My bad, I was/am looking at tiresize.com. The tire is a Trail Grabber M/T 35-12.5R/17 and what i am seeing is 597 rev/mile. So maybe I need to do as you suggest.