Alright here's how it goes.

I need 2 tires (because the car needs alignment which will be done) but I can't find the size I have on now on island. I run 235/45/17 on IX BBS wheels, Toyo R1Rs.

All I can get is 245/40/17 in the R1Rs, I would just change all 4 if that's the case because i'll be able to pay down on them.

I do a bit of autocross, maybe once a month max, what do you guys think about the 245/45/17 fitment compared to the 235/45/17?

 

I'm running 245/45 Kumho Ecsta XS'. wanted larger diameter for highway cruising & slightly lower rpm at the top of 4th at the road course I usually run. Little bit of rubbing on the inner fender at full lock, but nothing dramatic.

 

Is that on stock height or lowered? My car is lowered a bit

 

I run OEM 235's on my street tires. But for autox and track events I use 245/40r17's on the OEM wheel. But tires are different. In that size the my Kumho V710's were about 1/2 wider than a Dunlop Z1 star spec. Either way do some research on tirerack.com they list specs on tire widths. You should be fine with a 245/40r17 R1R.

 

RCE springs so slightly lowered as well.

 

The 245/40's will work fine, but don't mix the sizes. Diameters are different and you could mess up your drive train.

 

if you run 245/45's and you autocross the car, you will rip your fender lining out...ask me how I know. 245/40 works, but it's a little off. the best upgraded size for stock wheels is 255/40 if you can get them.

 

Have you thought about ordering what you want from tire rack? Yeah, I know there is shipping involved, but it solves the immediate issue of size and varying opinios..

 

Not true. Depends on what suspension you have.

I ran on Swift SpecRs with 245/45R17

 

I checked Tire Rack yes... I think I will just buy 4 245/40/17's if they have them in stock here on island. Duties always kill us when importing items so we try to fInd what we can on island..

I won't be mixing tire sizes for sure.. I know the 245/40 shouldnt rub height wise but not sure about width wise? I'm on Fortune Auto 500s so I can adjust if needed also. This is the height of my car:

 

good point. I was lower than swift specR's, and 245/45's didn't work well for me at all. 255/40's worked perfectly, and I've run that size ever since. The 245/45's didn't even look right, and i didn't like the way they felt. Too much sidewall.

 

Lowered cars require careful fitment choices.

Sidewall differences:
110 mm (4.3'') to 102 mm (4'').

IMO, could have been other factors or changes. Can you 100% say 0.3in difference in sidewall was the causal explanation?

 

nope...it was height coupled with the stiffness of the suspension, but almost a half inch difference makes a difference lol I never had an issue with the 255/40

 

8mm is hardly a difference.

Being that lowered, I be you were only *just* off the fenders.

 

i'm on gtworx with cooper rs3-s in stock size and rubbed on auto x-ing