autox 17x7.5 tire size
 

s13 with 03' z33 wheels f/17x7.5 r/17x8.
needing new tires and want to know if a 235/40 would be to much for the 7.5 wheel. I know it will fit and that it will bulge slightly, and that bulging can cause less contact patch. researched and cant seem to find anything pertaining to how it performed at an actual autox, just people saying they thats all they run on their daily with 7.5. google found couple bmw threads where e36 came factory with 235 on 7.5, but of coarse 240 and m3 arent the same. Would I be better off just going with a 225/45.

side note: this is local autox and dont care about classing.

Looking at a tire's spec sheet can be helpful. Here's one for the Direzza ZII: Dunlop Direzza ZII

You will see that 235/40 is not approved for 7.5" wide. Although you could run 235/45 or 245/45. Take a look in the wheel fitment thread, as your offest will dictate how much clearance you have to the coilover/strut and therefore the tire width you can run.

Grassroots Motorsports Magazine did a test on this a few years ago. They ran continuously wider tires on the same size wheels and recorded the results and driver impression. According to their testing wider is faster, PERIOD. Every time they went wider the car went faster. The downside is that the driver commented that it started feeling less responsive. I would say that 225/45 on the 7.5" wheels and 235/40 on the 8" wheels should be ok. I would definitely recommend that you get a set of non-staggered wheels in the long run though. Unless you're pushing big power there is no need for staggered tires.

^^^^

What he said. Learn on a simple balance setup first off.

thanks for replies.
yeah i knew the spec sheets recommended 8" min for 235 was wondering how much of a negative affect it would actually have, and I have the clearance for 235 up front.
Was also wondering if being square or staggered would be better(assumed square would be the answer there) so thanks there too. I didnt plan on autox when i originally bought(plus they were super cheap at that time). going to go with the 235 and ill start looking for a square set up, ill find new wheels long before tires are gone

It will look funny, but it may be beneficial to run the 7.5" wheels in the back w/235's all around. The additional sidewall flex may help keep the rear end in line and the square sidewalls in front will help with steering response. Just something to play around with...

ah true i never thought about trying that. either way doing it for fun not really trying to be competitive

So does having a staggered setup "hurt" or is it just that there is no need for it? I am currently running the same wheels (8 rear 7.5 fronts) and I have 245/40 in the rear and 225/45 in the fronts, but I don't have enough seat time to have noticed the differences from previous setup. I may be getting new tires in the next year so i'd like to optimize my choice in size.

Wider tires in the rear means that the rear has more available grip. This makes the car tend to understeer off the throttle. The only reason to run staggered tires is if you have enough HP that you have trouble keeping the rear end in line on the throttle.

I would suggest a 225/45 front 235/40 rear setup. Since the 225/45's are a little taller the contact patch will be a little longer. With the shorter 235/40's in the rear this should have a pretty balanced feel without looking funny or promoting too much off-throttle understeer.