Hey all, I thought before I worked out all the math I'd ask on here. I have a lot going on so it will be a while before I can sit down and mess with this.

I went from a 265/75/16 to a 315/75/16.
The truck has a 3.73 gear.

I can correct the tire size with my x4 tuner, but it has to be done in Revs/mile.
Has anyone else already done this for those sizes or am I going to have to suck it up and do it myself.

 

...........

 

When I had mine done by the dealer the tech was having some issues. He asked me if i knew the revs per mile and I did. That was the best way to recalibrate it seems.

You can easily find the info on Tirerack.com.

 

Let me know what brand tire you have and I will check for you. You will be right at 600 revs per mile, will vary some by tread, brand etc. But for the speedo I believe 600 will get you wear you need to be.

 

The tires are mastercraft mxt, 315/75/16

 

That looks to be right on the money at 599 revs per mile.

 

I'll give that one a shot! I used 580 and was 5 mph off.
I really appreciate it.

 

I would expect that to be off. 20 revs a mile is basically an inch in tire diameter. I think 599 will put you spot on.

 

Looks like I'm going to have to call SCT. No matter what rev/mile I put in, or the stock tune, nothing at all changes.

 

A lot of tuners don't adjust for tire size, SCT being one of them. Supposibly they don't want to change anything with the ABS system. Though a small number have been known to be able to be adjusted with the SCT, it may only be a certain strategy codes. The dealer told my dad they can adjust up to 35's when he had his 03' reflashed. My 04' is actually more accurate than factory with 33's than the stock tires that came on the truck.

 

There is a 3% variance for the speedometer right from the factory. It is hard to be gps matched all the way across the speed range. As you speed up the gap will get worse since you are working with a percentage. 3% at 100mph could be 97 or 103, so at 40mph speedo would read 39 or 41 which you would most likely never even notice.
So getting within a few percent you should be able to call it good. But 5 to 10 mph variance at highway speed is a bit much. Hope that SCT can get you squared away.

 

I talked to SCT briefly, but since I'm at work and my truck is at home, I have to wait and get back there so we can try a few things.
He did seem like it should be able to be set with the box though.

 

Welp, no go on my truck.
The revs / mile is only for the shift points.
The speedometer is cable driven, not through the PCM.
Have to figure something else out.

 

If it is a cable driven speedo, then there should be a gear in the t-case. You should be able to find a different speedo gear to get you closer (did that in my Jeeps). But for a few mph off, may not be worth the effort. Proper shift points is for sure more important.

 

Need to look at the system myself.
My understanding is that they are 2 different and independent systems.
The tuner fixes the shift points.
The gear fixes the speedo.

So my shift points should be correct at 600 revs / mile, but I need to find a speedo gear.