So my stock tire size is 265/70r17 but I've had 275/55r20's for almost 2 years and my speedo has read 4 under since and no big deal..

This is before plugging in the tuner.

When using the tuner I left the axle ratio and tire size stock and my speedo still read 4 under according to gps and roadside radars.

But when I changed my revs per mile to 630 on the tuner, my speedo now reads 10 over.

According to tire calculators, my stock tire revs per mile are 638.4 and my 20's are 632.4...

Has anyone else had this issue?

BTW, I have a 4r70w and I swapped from high stall to low stall. I'm wondering if this had an effect???

 

You need to use trial and error to dial in the revs per mile.

 

I have the x4 as well and have noted similar issues. my tires should be doing 608 revs per mile. The x4 gives the option of 610 or 605. I selected the 610, and my speedo from what i can tell is reading about 10mph fast. 70mph zones in GA where everybody normally runs 70 or less I am getting passed with my speedo reading 80 right now. I guess I'm going to adjust it to 615 revs and see if that slows the speedo down to normal.

 

X2.
The calculator is a good number to start with but you have to fine tune it. When I loaded my tune on my x3, I had to fine tune it too. It has to do with the wear and inflation of the tires I think.

 

Well I have went to several different calculators and discovered they are all different. One site says my tire revolves 615 revs per mile. The manufacturer say it revolves 608 revs per mile. 1010 tires says it revolves 627 revs per mile. Seeing that at 610 revs per mile my speedo reads 80 when i am being passed by everyone in a 70 mph zone, I recalculated my x4 to show 625 revs per mile. We will see if that is closer to the real speed I'm traveling.

 

I got mine within a half of a mph after 4 or 5 times.