PDX-944

Hey, I can afford to have two of the wheels widened, but im not rich... lol
I read around a great deal beforehand and plenty of people had success with 295 on a 10 inch wheel. Not to mention I had run 315s in the rear on 10s the year before just fine as well.
I bought 335s originally, but setting them next to the car I just laughed, and sold them a day later for more than I paid for them.

Dash01

What makes you think wider wheels/tires are necessarily better, or improve handling and agility? Bigger wheels and tires will add lots of unsprung wheel weight, degrading handling and agility, since more mass means more energy required to spin up, slow down, and change direction, per basic laws of physics. And, there's the matter of how much mass the car suspension is designed to handle, and whether such suspension may be over-taxed. What you absolutely don't want is a light car wearing heavy tires/wheels.

Years ago on this forum a guy (as I recall named Mueller, a retired Coast Guard captain) who races his Porsches around cones on Saturday parking lot meets, and reported his experience with an 944 S2 or 911. Up to then, he'd gone with ever wider tires/wheels, ostensibly for better grip. One Saturday, his racing tires were inop for some reason, so for lack of other big tires/wheels he simply bolted on the old factory wheels/tires to race around the cones: He was amazed how much better the factory setup was, and how nimbly the car performed, simply because the unsprung wheel weight as restored to its designed specifications.

Bottom line: If you are not reducing unsprung wheel weight, almost certainly you are degrading performance, not improving it, all else such as tread grip being equal. Yes, your wider tires may grip better than otherwise narrower tires might, but the wider tires will also be heavier, as will the wheel and suspension work to support such wider tires, offsetting or perhaps negating any gains from wider tread.

JustinL

Some important factors to consider and data points for the conversation:
Do the tires exist and what is the selection like? I love some wide 16X9 club sports... but they're absolutely useless as a street wheel because no decent tires exist. Lots of race tire options though, so they make a great track wheel.
Will the wheels clear the brakes? Important to think about if you've got big brakes. Going bigger wheel diameter than the minimum to clear the brakes looks fantastic, but is probably excessive and the increased weight may reduce performance.
For a street wheel, I want it to look awesome and perform decently. For a track wheel I want it to look ok and perform awesome. I have a lot of wheels for my cars and each are good in different respects.
16X9 square clubsports with 245/45/16 hankook z214s: My fastest lap times are on these and also fit the rules of maximum tire width for the class. They don't look great, but still pretty hot.
17X8.5, 17X10 kinesis k27 with Toyo RR 245/275: Also very fast combination and easier to drive than the square club sports. They look pretty good when cleaned from brake dust and polished lip.
18X10 square Toyo motorsports forged with 255/285 street tires: Capable on track, but slower, even with r-compound rubber... but my word they look so hot!

Track times are all on my S3 race car, so with more power and weight from a 951 there may be more advantage in going with wider.

Cloud9...68

All great points - unsprung weight of the wheel/tire combination is an important consideration, especially on a more "power-challenged" car. And everybody has different goals, and drives on different style tracks. I wanted the widest wheels I could fit without having to add spacers or roll my fenders, or to add unsprung weight. The 17 x 10.5 Signature SV103's I have on order weigh about 3 lb less than my current 17 x 9" Twists, although the tires will be slightly heavier (by about a pound), so I'll call that a wash. My fastest times so far have come with 225/45-17's (BFG Rival S's or Nitto NT01's) on my Twists, and my car is very much grip-limited, so I'm salivating at the thought of an extra 1.5" of width at each corner. The track I do most of my driving on is very tight and technical, so the extra rolling resistance of the wider tires shouldn't be significant.

PDX-944

first things first with regards to "grip-limited", is to determine if that is oversteer, understeer, or if the balance/driving is fairly spot on. only you can determine that at this point since we aren't there.
IF the balance and driving is good, then you can look into upping size, or perhaps finding a better tire like an R7. If youre driving on the track, your tires don't have near as hard of a time heating up as they do in autocross or hillclimbs which are shorter sprints. With decent power, I think youll notice small changes in wheel weight less than other things. 1.5" more is a huge jump by the way haha, perhaps a smaller increment first, which might fit easily onto a wheel setup you have now, and then determining if you need to do anything special to really get more.
you can get as big of tires as you want, but if youre on a street compound, youre on a street compound. are you in some sort of class that prohibits DOT R comps?

Cloud9...68

The car was aligned, corner balanced, and tuned by a very experienced track car prep guy, and it is about as neutral as it can be. When I say it's grip limited, I mean I can feel it losing grip during steady-state cornering, when there is minimal driver input or skill in play. But that's about all I want to say about this topic, because these discussions tend to quickly spin into debates over spring rates, roll stiffness relationships, ride height, alignment settings, tire pressures, driver technique, and on and on and on, which are very difficult to diagnose over a forum discussion. Suffice it to say that my car's suspension has been upgraded with all the best available components, and I have enormous 944s 4-piston calipers, so it's clear the suspension and brakes are writing checks the tires can't cash. That should change very soon.

Jllewan

Update -

1988 951

265/35 x 18 Kumho's on 10.5 x 18 ET60 Carrera IV wheels on back
245/35 x 18 Kumho's on 8 x 18 ET 57 Carrera IV wheels on front

Got a nice closeout deal on the tires from tire rack $400 (why I went with those sizes)

Perfect fit on all stock suspension.

Voith

http://wheeldynamics.com/appguide.php

jerome951

I'll share a bit of my experience.
Wider isn't always faster. I used to run 255x17 front and 275x17 rear on the track. 275 on the front wouldn't fit due to class restrictions on rim width. Had to fiddle w/ sway bars (really stiffen the rear) to get the car balance to be neutral.
For the heck of it I tried a 255 square setup. I gained a few tenths on a 1:26 lap. The larger rear diameter was reducing acceleration because I was limited on front end grip.

If you really want to maximize grip, run a softer tire on the track. A good street tire vs. a harder track tire (like a Toyo RA1) is about 2 sec/lap difference. Difference between an RA1 and a Hoosier S7 is also about 2 sec/lap

mbean