Wheel/tire Fitment Tool? Ever Used This?

[FastDriver]

http://www.jegs.com/i/Percy's/760/01201/10002/-1?CAWELAID=1710534765

I'm in need of some way to measure out the biggest tire I can fit on the rear of my car. I have 17x12" weld S71s, and there's a lot of room left.

I want to drop to a 15x?" wheels and put the biggest slick I can under there. The Weld RT-S S71 wheels are $500 each. So I definitely want to get it right the first time. I was thinking about using the tool above, but common poor reviews say that the plastic is cheap and the measurements are not very good. Also that the wire that represents the tire is not easy to set unless you have the tire you want to use, in which case, what's the point? Have any of you used this or something else? Does anyone have a better method?

Clearance issues I foresee:
- 12" rotors and brake calipers are not going to fit most 15" wheels, but the S71 has more clearance than most. I will find and test fit one before ordering. I may grind down the caliper, or drop to 11" rotors/brackets as necessary.
- Bias ply sidewalls bulge substantially more than radials. Mickey Thompson's website calls for leaving an extra inch of clearance on each side.
- I have a buddy with a fender lip roller that I plan to use to add a bit more room on the outside.

 

[90sickfox]

I've used the Percy wheelrite tool. They are right about it feeling cheap and plastic.... But it works. The tire wire is a pain and almost useless.

I just measure from center to top of wire and get the wheel and tire diameter measurement. A quick Google search and the tire wheel combo is found pretty easy.

If you know you have a wheel that's a 18 x 10.5 inches wide with a 45mm backspace and need a combo of 27 inches. This will help calculate...https://tiresize.com/calculator/

 

[FastDriver]

That's what I was hoping the tool would tell me. I'm not sure exactly how much tire I can fit width-wise in the back. I'd like to order the wheels and slicks based on measurements.