I just put 285/70/17 Nitto Trail Grapps on my 12 Sahara. My speedo is matching up within 3mph of my iPhone GPS app.
My question is will the Procal actually do me any good other than turning off the TPMS light?

I have the auto trans.

 

It will also correct your auto trans shift points for the larger tire size.

 

I thought that it only changed the one variable, wheel size. This has an indirect effect on how the auto shifts as it is one of the factors affecting shift timing. But if the OP started with 32" tires, I doubt it would make much of a difference.

 

I feel like it made a difference for me but I have an '11.

 

If one was to install larger tires AND re-gear, ProCal would be the "cherry on top," but not needed

 

go ahead and pick one up. The ProCal is a fantastic little tool with great functionality. They are fairly cheap too.

It will adjust your shift points, get your speedo in check and turn off your TPMS. It can also set proper gearing (if u choose to regear) along with a few other things.

Really can't beat it for $150 IMO

 

I just went through this, auto 13', I drove the vehcile for a bit over a week without making any adjustment. Trans shift points were a bit fuzzy, shifted too late, or not at all when it should have, not bad, vehicle felt labored at times, not to the point where it wasnt drivable , but just enough to be noticable to me. Changing the tire size input helped the trans understand what was going on. With my driving style, the trans shift points were just abit off without the recalibration. Drivability for me is much better with the correct tire size input for the CPM.
It took maybe 5 minutes to measure the tires, and reprogram with the Procal. Plus I set the TPMS cutoffs. and the unit helped me get the steering wheel straight after my spacer lift install.