I've been looking on some of the previous threads and using the search function and I may have missed the answer and if so I apologize. I'm going to need rear tires soon. I'm running stock 18's with Pirelli's. 225/40 and 295/30. The fronts have more to give and I'm okay getting rears and then trying something else. My question is can I run 315's on the rear with the 225's up front? I've read that 235 & 315 works. I've got a lot to learn and this is a great place to get some great answers. Love the look of the 315's in the rear. Options? Thanks.

 

225 and 315 doesn't matter, thats more width of the tire.


its the 25/30/35/40 that matters (height). Run a 25 to 30 rear and 30 to 35 up front and you should be fine. I forget the exact percentage needed to not affect the diff. Some people say its a farce but most people follow the rule of thumb.

 

Thanks. I knew it is wider than stock. Height, got that as well. Just never heard any talk about this set up before.

 

pretty sure that 3% difference in rolling diameter limit is not myth, but a rule to be followed. running the 225/40 will not cause differential issues, but you'll probably see abs/psm cels until the new rears get scrubbed. not to worry, the car is very sensitive to new tires against old(er) ones *intially*.

me, i would get rid of the oem pirelli's owing to their age, assuming they are still from the stock fitment(?). if not?, the assyms are a very solid tire choice for DD'ing.

 

I was looking at the paper work last night and the tires were installed in 2010. No mileage was indicated at the time which I thought was strange. Four years old? All of my other paper work is spot on and the car is in great shape with only 20,000 when I bought it. I've put almost 3,000 on the car since November. Love to drive it. So now if I change all four I can go to 235/335. Then the tire dilemma begins! Choices?

 

yes u can do that with no issues.

 

All three number in the tire size matter because the aspect ratio is a percentage of the width. So a 305/30 will have a smaller diameter than a 315/30.

OP, your best bet is to check the tire spec sheets from the manufacturer for diameter, then try to keep it within 3% F and R with the front slightly taller. To understand the reason for this fully real about viscous coupling awd. The tire diameter given by a manufacturer may not be the diameter you will see because the width of the rim used will effect the actual diameter.

 

I'm on stock 18's I believe. So 8's and 11's. I was thinking of changing wheels maybe as well. If I go with the 235/315 on new wheels then should I go to 8.5-9's on the front and 12's on the rear? Then offsets? New territory for me. I've read about the 3% difference and will be sure to keep it that way. Steve K it was your post that made me decide to go to 235/315. I've read many posts about this but still I'm a slow learner.

 

Looks like my only choice in these sizes in PS2's and Pirelli P Rosso's. So, then this begs the question of going to 19's? I knew this would not be easy...... but fun!!

 

The big reason I am running 19's is because of the tire choices. I wanted to run Michelin Pilot Super Sports and they weren't available in 18's in the appropriate widths.

 

So in doing some more research it looks like the Pirelli Rosso's in 315 are my only choice if I stay with my P-Zero fronts. Any problem with the different tread design running Zero's up front and Rosso's in the rear?

 

You do not want to go to the 315 with half worn front tires in that size. You need to stick with the 295. It would require a 315/20 to get the rolling diameter within spec.

Yes it is absolutely a rule to be followed. People (including myself multiple times) have even had issues when getting over the 2% mark.

Was the 335 a typo given your post below about the 315, or did you change due to the lack of 335 tires for 18" rims? Either way, the 335 is more tire than you need. That's personal opinion, I think when you go with that much of a difference between the front and rear tires it introduces some weird handling characteristics, especially at the limits. It can (and likely will) introduce rubbing issues, so I'd stick with the 315 anyway, even though rubbing issues are more limited when you're not lowered.

I've run 295, 305, and 315 rears and definitely notice the additional grip of the 305, and the extra grip of the 315 beyond that, but have gone to focusing much more on tire performance/tread/compound over size. Personally I now run Toyo R888 semi-slicks/rain track tires on the street as my summer tires, and as my track tires too. They're the stock size 225/40 and 295/35 on a set of 18" OEM hollow spoke Turbo twists I picked up from another member on the board here. We don't get much rain in CO summers so the lack of water channels doesn't really affect me. I may go up to a 235/305 combo for my next set, but in the stock size I already get a lot of grip even with my upgrades (I'm at ~600 hp/tq).

You can run the 235/315 on your stock wheels ok, though the 315 is the max for that rim width, and it's going to look a little funny with sidewalls that are a little off vertical. 245 is the max for the stock fronts. I wouldn't want to run that combo at the track, but it's fine for the street.

Personally I think 19" is the ideal size for the Turbo purely in terms of looks...

There are two schools of thought on this. Some people say as long as the rolling diameters and tread compounds match, you're fine (some say the tread pattern needs to be similar too). The other side of the school says you need to match the actual tires due to the sensitive Porsche center diff. Personally, given my experience with PSM across a half dozen sets of tires in both 18" and 19", I would stick with the same tread pattern.

One last factor to consider is different model tires of the same listed width aren't necessarily the same actual width, so without actually measuring the tires, you can't compute an accurate difference in rolling diameters. Normally each manufacturer will use the same standards for measuring the widths, especially for the same class of tires (high-perf summer, all-season, winter, etc), so your risk can be limited, but that's not always the case.

 

Scott, thanks for the thorough answer! I love it. I'm thinking the best way to go is to replace all four tires. I want more width on the rear's and in researching on Tire Rack I found that the Advan AD08R in 295 gives me the width I am looking for. The tires have gotten some nice reviews. I'm not planning on tracking the car anytime soon so just some spirited canyons on the weekends. Tire diameters are the same for F/R.

 

FWIW, the new Pilot Super Sports have been getting all of the best reviews by far of any of the tires available for our cars. If I still ran street tires at all, those would be my choice for sure.

 

To bad they don't have the 295/30/18's. Tire Rack only goes to 285's. I know the reviews have been great.


Jim