Not that I am running a staggered setup (285/40-18 X 255/45-18) what tire size do I put in when requesting new tunes?
I assume the front tires due to the speed sensor? Not sure though....

 

You want to find out the tire revolution for your specific tire. Each brand is different even in the same size.

 

I always assumed the rear tire size as I think the speed sensor is located where it can read the transmission output shaft? but I don't know for sure. Good question.

 

In your case, it doesn't matter which size you enter, as both tire diameters are virtually identical, less than 1% difference.

 

That makes it easy, what if they weren't that close?

 

Reading will be based on the rear tire.

 

Ok so I just put in an order for new tunes from bama, it asks for REAR tire size.
Guess I never noticed that before.

 

Rear tire size diameter is the issue. I used my GPS to dial it in exactly by mph. Even the tuner setting was off when I put on the new tires by putting in the new diameter. I had to do a little trial & error. Speedo is now 100% correct.