I currently do DE events and am considering changing my tire size on the stock Turbo wheels. I have read about people increasing the rears to 315s and the fronts to 235s, but I currently only need rear tires. Would it create a problem going with the 315s in the rear for now with the stock 225s in the front or do I need to upgrade front and rear at the same time due to the all wheel drive of the Turbo? Also, with stock suspension, is there any rubbing with the 315s in the rear?

 

I'm not sure the stock rear wheels is a good idea for 315. I'm sure you can make it work, but probably best to stick to 295 for DE events. The car understeers enough as it is.

Having said that, if you want to run even wider rears, just be prepared for even more understeer.

I have heard people running 315s but I don't believe they were on stock suspensions or alignment. Some even had rolled fenders.

Bigger is not always better...make sure you know what you want the car to do.

 

Thanks for your reply speedyellow. I definitely do not want to increase understeer. My main concern with the 295s that I currently have is that they almost seem stretched in the fact that the sidewall seems to go from wide at the wheel to a little narrower at the tire edge. This has resulted in a rolling effect and strange wear when doing the events.

 

before you change tire sizes, you may want to check your tire pressures. what kind of tire are you running?

 

I currently have the PS2s. I have been told by several instructors running similar cars and tires that I should maintain 40 lbs hot all the way around.

 

In that case, are the instructors also getting abnormal wear and that 'rolling effect'? In may not be a tire issue but something else. Also, most instructors in my area don't run cars with street tires, so I find it interesting that they are on streets in your neck of the woods!

personally, i wouldn't run them on the 315s if I was on a stock suspension and wheels. I don't think it'll be a major factor.

Are you fairly experienced or are you on the newer side at the track? Just asking since there's probably more on the table to be gained by monitoring tire pressures (is 40lbs hot all the way around really working for your driving style) and possible alignment than changing tire sizes on a wheel that is probably on the narrow side for a 315 tire (along with possibly needing some fender rolling or fender liner mods).

 

You are correct as far as the instructors are concerned. What I meant is that they had run a similar combination at some point prior to upgrading their cars. I am now an intermediate driver, so I am still gaining seat time and trying to gain knowledge from those around me.

 

Thanks. I actually instruct and sometimes I do run on street tires to get back to what beginner/intermediate students deal with. Unfortunately, I haven't used the 996TT at the track so feedback from personal experience just yet.

I would advise weighing the pros/cons of doing the tire upgrade. My opinion (and I'm sure others will have different ones) would be that it may not be worth the trouble to do so considering the negatives above, and it also may not address the problem that the change was trying to resolve.

Another instructor actually runs 235 tires up front and 295 rear, to get better turn in...

 

My favorite on-track street tires are Bridgestone So2's 235/295.... The sidewall stifness is the main plus for me... I run them at 38 hot....

On the stock wheels I would stick with the 295's in the rear... If you also drive the car on the street consider buying a cheap set of track wheels and run whatever tire/size you prefer.... keep the OEM wheels for street use and just replace the rear tires with what you are currently using

 

First back to your initial question. If you are changing (or considering) going to 315 rears you will need to change the fronts so the overall diameter is about the same (as you thought about the all wheel drive and PSM)....hence the need to go to 235/40/18 with 315/30/18. It is certainly possible to run this combination (I run this) and am quite happy with it. I run MPSC and the air pressure is quite important...most keep it well below 40psi hot!..Do a search as this topic has been covered ad-naseum. Should have no rubbing with stock suspension (4X4 height)...I run mine on PSS9 lowered by 3/4-1 inch.

 

Do you run those tires on stock TT wheels or GT2 wheels?

 

Stock size rims!

 

The above description of how the tire is wider at the wheel level and tapers to the tread area is exactly how a well fitting tire wheel combo is supposed to be. This creates less side wall roll. Look at any race car wheel tire set up and you will notice the same set up.

 

You tell 'em Tom!

 

A 315/225 would probably have too much diameter difference and might heat up the fluid coupling. 315's are about 25.5" diameter new and your used 225's are maybe 24.7".
I've run 235/315 Rossos and PS2's on stock rims with stock suspension. The wider tires ride smoother, tolerate lower air pressure, are more resistant to pothole damage, and the slightly bulging sidewall protects the rim from curb scratches. Mine don't rub my non rolled fenders. The fatter tires look good and fill up the wheel well more. The turn in is good and break away is smooth, compared to OEM Pzeroes.. That may be due to the PS2 tires more than the size though. I have not compared them on the track. I thought about running 335/245's on GT2 rims, but I'm not sure if the heavier GT2 rims would be worth it.