RTR tire sizing
 

Probably going with Z2 Star Specs. What's the best size to use on a 7" wheel for RTR class? Was thinking of 235/40/17 or 225/45/17. What do you experts think will be faster?

The FR-S I drove in RTR recently was on the 235/40-17 ZII's, & it seemed to work very well. From what I have heard, that diameter ends up with about 58-59 MPH in 2nd gear, but if you run on courses that have longer straights that may not be enough.

I run the 225/45-17 size on my 6-speed NC Miata, with my short gearing that size helps extend 2nd gear which was more important to me than an extra 10mm in width that may or may not be effective on a 7" wheel. With the soft rev limiter in the FR-S, I would think your decision would be more influenced by tire diameter than by width, as hitting the limiter is just too costly in these cars.

Yeah it's a tossup between 235/40/17 and 225/45/17. I'm a fan of the 235 in most instances, I don't think 225 in reality really buys you much gearing wise at the top. I don't think you can go wrong with either one though.

I was thinking 235/45 to extend second, but that's quite a squeeze on a 7...

It will fit no problem, we have a local competitor with a CS Miata MS-R (2010 National Championship car) that has experimented with 245/40-17 Star Specs on a 17x7, the issue is whether that extra width is effective over a 225. With street tires, there have been many reports of people trying really wide tires on narrow rims & not going really any faster. Plus, a lot of people say how much more responsive the narrow set-up is.

For those that don't know, Andy Hollis is a long time autocrosser with some National Championships under his belt, he hooks up with Grassroots Motorsports every year for tire testing. Check here for some great info on street tires in general, including results from testing all the hot tires from the past few years, & here for part two, including initial information on the latest tires.

The 235/40-17 is 24.4" tall, the 225/45-17 is 25.0", & the 235/45-17 is 25.4". The car-owner of the FR-S I drove in RTR did experiment with the 235/45-17 last year, I'm not sure if he is on this site but I will ask him what he thought. IIRC, he didn't like the response from the taller sidewall, & didn't like the lack of grunt coming off slower corners. If that's the case, the 225/45-17 may be the best bet...that diameter should get just over 60 MPH in 2nd gear with the gearing in the BRZ/FR-S.

Personally, I'd rather have a little more gearing advantage from a smaller tire than having more top speed from a bigger tire. If it's a very fast course, you can switch it to third. If its moderately fast, you can ride the rev limiter. You may or may not need to stretch 2nd gear, but you will always want more gearing advantage.

I'd go with 235/40 personally. I would be concerned that the inside of the tire rubbing on the front strut if I maxed out the front camber bolts.

I'll be running a 42 offset wheel. With crash bolts probably lucky to get -1 to -1.5 camber. I was thinking there wouldn't be any clearance issues. Am I wrong?

Rubbing isn't really an issue with the 235s, you can't get that much camber out of the bolts usually. If you are +42 you have nothing to worry about. I'm using 245/40 Hoosiers with +42 and it's not an issue.

You can't ride the rev limiter in this car. It has a soft fuel cut rev limiter which basically kills all the power in the car. What you can do is hold the RPM steady. Or shift of course.

The 235/40 Z1's worked well at Nationals though I don't really have much to compare them with. Only had a couple runs on John's Yokohama tires which were well seasoned before we switched to the fresh Z1's and stayed there due to how much quicker they were. I don't remember what size for sure the Yokohama tires were. We had the Z1's on 50mm rims (Kosei K4) and had no issues with rubbing. Camber bolts were installed at max negative up front. I plan to stick with the 235/40 until I see results that would suggest a better choice.

Great info in here. It's leaning to either the 215/45 or 225/45 Z2 star spec for me. I wonder if the cars lack of power will be better suited to the 215's...?

I went with the stock size 215/45 Star Specs for mine last year. Wasn't planning to try and be super-competitive in RTR, just have a good tire for spirited street driving and to run in RTR locally on occasion. If I had to do it over again, I'd probably go with the 225/45's.