I replaced my stock Rubicon tires /wheels with 33x12.5-15" GYW Duratracs on 15x8 MB wheels. I'm running my stock tire size setting with the Procal and my speedo is pretty close, <1% accuracy using GPS. I tried other settings, 32.5, 32.75, and speedo was off so I set it back to stock and just disable the TPMS with the Procal unit, which is the only reason I bought it, until I go up to 35's eventually ( I know, I should've just bought 35's from the getgo, LOL!)

The Procal is also good for centering steering wheel after lift install, and for fast idle when winching, and other features....

 

If you have a GPS test the speed and mileage. I did mine on a 20 mile commute to get it dialed in right. Put 35's on but had procal set to 33.25 to get it correct. The other features are nice as well it's still worth the price in my opinion.

 

X2 on checking your speedo against a GPS. I went from 32" KM's to 33" Mickey Thompson MTZ's. I have the nagivation system, so it was easy for me to check the speedo. There was no change in the speedo between tire sizes, so I didn't bother buying anything to recalibrate.

And as far as measuring tires, they should be measured mounted on the wheel, but not mounted on the vehicle. Using this method, my 32" KM's measured 32", and my 33" MTZ's measured 33". Of course they will be shorter once on the vehicle. They can even vary in height on the same vehicle, depending on overall weight. I'm sure my tires would be shorter on a JKU with heavy add-ons, compared to my lightweight two-door with no heavy add-ons.

 

OP, I just went out an measured my stock almost full tread SR-As with 36psi in the tire. I got just a hair over 31"s. If your Duratracs measured 31"s, I would change nothing.

Note: Mine is an Unlimited, dont remember what yours is. Weight can be a slight issue, mine is all stock.

 

Thanks for the replies it does help. I was hoping that the procal would help with the constant shifting on the eway but if new tires and wheels are that close then I guess it wouldn't make a difference with my current set up.

 

That's interesting, because I had been considering going with this setup from my current 275/65/R18 Duratracs on stock rims. I measure these to be just under 32" installed and sitting on the ground, so I will either save my money and stay with my current wheels and tires (and replace 2 at a time instead of 5 at a time, much easier to swallow) or go to 35's like everyone says. 33's that measure 31 don't interest me at all!!!