I've read where others have replaced their OEM size (295/30-18) with 315/30R-18 rear tires. Is there any disadvantage to doing so? Is there a justifiable advantage to doing so? I need to replace my rears ('04 GT3) and am considering this upgrade but I am unsure if it will cause any issues such as the potential to rub or limit alignment possibilities. At this point, the car is street driven only (my daily driver) but may participate in future track/autocross events.

Thanks for any input provided.
Joseph.

 

I have been running the Toyo Ra-1's 305/35 on my GT-3 rear the only issues are 1 that there is a small tab of metal just above the front leading edge of the rear bumper cover.(right where it lines up with the metal of the body .This must be bent out of the way or the outside edge of the tires will rub under heavy cornering loads 2 as you are running a wider tire you will have a somewhat wider sidewall.This will effectively soften the spring rate at the rear giving you slightly more grip in the rear and maybe a little more understeer.What I wound up with was the 245/40 toyos at the front and the 305/35's on the rear with my rear sway full stiff and my front sway one from full soft the car is very predictable.Hope this helps HOM

 

no abs issues either?
what about gearing?
i would love to run toyo's on mine.

 

No ABS issues I guess there is some slight change in gearing with the slightly larger diameter of the rears wouldn't that tend to effectively give you slightly lower gearing?I am not sure of my physics at all!

 

I ran the Kumho 710 315 & 245 .. it never rubed at the track but on the way home over large bumps it touched a little...nothing serious.. howevery this combo does not work for my ...ABS failure due to the difference in diameter front to rear ratio.

On the car now is the v710 245 & 285..car has great stick.. next setup waiting in my garage is the Hoosier 245 & 315.. R6.. you can also get them in the stock size of 295.. the front to rear diameter ratios are very close. Should work well.

Do a search and you will find NJ-GT has stuffed everything in his car..all the way up to a 335.

 

Yeah but he has always maintained the proper width ratio so that it didn't affect the balance. 315s on the back with the stock width tires (235s) on the front will cause it to understeer terribly, especially at an autocross.

 

HOM,
If my thinking is correct, I believe the taller tire will raise your overall gearing and shift towards faster speed but less grunt at lower speed. I definitely don't want to alter tire diameter but greater width is a possibility. This, as long as there are no adverse effects.

Larry,
According to the specs for the Michelin PS2, the 315 is .7" wider in section width. Do you feel this amount of difference will effect the under-steer that severely? Would a firmer rear torsion bar setting counter act this? Or, a softer front setting? Or both.

The other difference between the two tire sizes is the diameter. The 315 is .5" larger. Also, the 315 is 3 lbs. heavier. Hmm... I think I am leaning towards the 295 at this point. Anyone else have experience to share?

Joseph.

 

Not at all I been running Fiski's 18x12 and 315 and not problems. If any you might encounter that if you PUSH the car to the limits the suspencion ruber components tend to give up to much and makes the tires the rim rube on the inside.
I have all solid Monoball suspention and the problem is gone. You will need to play with your sway bars till you fine the car balance and feels good to you. In my case I am at factory on the back and full stiff on the front.

These works for me. Regarding gearing and all of that realy dont know. As for weight I will say that my Fikse's are way lighter than the factory wheels in the back and in the front. 2 weeks ago I weighted the car and I am at 3017lb with a full tank of gas.

Love the wheel just maje sure that the offsets are done by someone that has done these before because is very very tight in the back.

 

For the track, you can probably get away with the 315s (if they don't rub) because most of the guys that I know have softened their rear bars a hole or 2. I found that the car had too much oversteer when really run hard on street tires. With 400 hp, the difference in acceleration is negligible.

For autocrossing, you should definitely stick with the 295s. You will want the car to rotate as easily as possible, which it will not do with the 315s. Remember that NJ-GT had 265s on the front to get his car to turn in.

 

For the track, run 235/295 or 245/305 on the stock wheels. For autocross, go full stiff with the rear sway bar, and one from stiff for the front or softer until you get the balance you like.

I wouldn't run 235/315. The car has already plenty of understeer from the factory, so it just makes it worse.

For autocross I'm running 285/315 and for the track 265/315, but I'm also running wider wheels.

 

has anyone run 265/305 on the track?If so what sway bar settings?TIA HOM

 

WTF! It would be worth it to take a ride up to Edison just to see those bad boys stuffed uder a GT3! That 285 is my REAR tire!

 

NJ-GT,
What width and offset are you running on those wider wheels?
Did you have to go to extreme negative camber too?
Thanks, Pat

 

No need to drive that far Larry. Here are some pictures.

18"x10" ET33 on Kumho 285/30R18
18"x12" ET58 on Kumho 315/35R18

-2.5 degrees negative camber at all wheels.

This tire combination causes an error with the ABS computer, so this is not recommended for the track, only for autocross. For the track, 285/335 or 265/315.

 

Awesome. Looks like they were there right from the factory.