Ken7258

It seems 17" wheels/tires are a lot cheaper then 18's. If I have 235's in the fronts and 285's in the back on 18's or 17's isn't the contact patch the same. What am I giving up in performance and why? Thanks.

Ken

hinchcliffe

Depends on wheel width also. Tires will stretch out across a wider rim giving more patch.

ceboyd

i'm not sure on other cars but we used to have his and her's 993s and noticed that for track duty, the 18s are better (and made up to a 3 second difference on lap times)... although that is with stock sized 17s vs stock sized 18s.. well, 225 45 17s MPCS in front with 255 40 17 in back vs. 225 40 18 in front and 285 30 18 in back.... and there wasn't as much rollover on the 18s obviously.


If you do a search, you'll see it has been discussed at length and we are not the only ones that reached the conclusion of the 18s being better for 993...

I do NOT know if this is the same case for other model porsche's like 996 and 997 or 964 however... and perhaps 944 means a 17" is best...

mark kibort

lap times will be near identical for 17 vs 18s. thats kind of silly to think there would be a difference in total performance. However, there will be subtle differences, usually in areas of choosing pressures and handling characteristics.

we often go between 17s and 18s and the biggest factor is what condition the tires are in. In my first change to 18s 6 years ago, i kept the pressures the same and noticed some oversteer. quickly fixed by lowering the pressures by 2psi.
mk

George from MD

I run 18s on a 993 and really picked them because of the advantages of better turn in/feel etc. and I was concerned 17s would not fit over the bigger brakes I was planning on buying at the time.

But two friends and colleagues with virtually identical cars and not that much more experience regularly wax my tail at Potomac PCA events on 17 inch wheels and the same brand of (Hoosier) tires. I can't blame the wheels and would suspect if you're just doing DEs you would be perfectly happy with 17s and would put your money to better use getting more experience.

ceboyd

o.k. so then I guess I wasted money on using 18s instead of 17s.... but driving 17s vs. 18s back to back on a 993 showed me that my personal preferance was 18s ...thus I use 18s

..and I like being able to have factory stock 285 30 on 10" rears vs. the 9" rear 17s ...and I like the 7.5 or 8" wide front wheels instead of the factory 7" where a 225 45 looks buldged

Bill L Seifert

I'd be real careful putting 18's on a car the Porsche does not recommend 18's for. I can't for the life of me see what difference it would make if the outside diameter was the same and nothing rubbed. But Porsche does not recommend 18's for some 993's. They know a heck of a lot more than I do.

My car has nothing in common with a 993, but I can't tell the difference between 16's with 245-45-16's and 15's with 245-50-15's. But that's just me.

Bill Seifert

1983 944 Race Car
(also have a Boxster with 17's)

Garen

I would think that it would be a combination of tire brand, compound, sidewall stiffness, tire width, overall wheel-tire weight, and suspension setup. Generally, a wider tire would provide more grip. That said, there are a couple of 930 owners that swear their cars feel much nimbler with the original 16"'s when compared to aftermarket 17" or 18" wheels. Don't forget that F1 cars run 13" wheels, as dictated by the rules.

If you are dealing with the same width and compund tire in 18" and 17", it would make sense that if the wheels are of the same type of construction (cast vs. cast, modular vs. modular, etc) that the 17" wheel setup would be lighter. This would help unsprung weight, which is always a good thing. The 18" setup would, potentially, provide better turn-in due to less sidewall flex (remember, same tire model and width, just different wheel diameter).

That said, I would bet that a set of 17" wheels with DOT R tires would be faster than a set of 18" wheels with street max-performance tires

I for one absolutely loved upgrading to really wide 17's on my 930. I run 17x11 rears with 315 Victoracers, and 17x9 fronts with 255's, and couldn't be happier. I call them my "insurance policy".

Just my novice $0.02.

Garen (87 930)

George from MD

Ceboyd I don't think you or anyone would really waste money (of course my wife says this entire enterprise is a waste of money).

I think the 18s look a lot better than the 17s and for that reason alone it could be worth the $ difference to a lot of folks. Having driven both even I can feel a (slight) difference. But I suspect any real performance difference is pretty negligible.

BTW to my knowledge 18's were not prescribed or approved by Porsche for 1995 and early 96 993's (non turbo) without reinforcement of the steering rack. All later 993s have the part already.

ceboyd

yes, my 96 already had the option of 18" from the factory (and the sticker inside my door has pressure options for 18s).. and I DO indeed have the steering rack brace

forklift

There was a thread on this a month or two ago and the consensus was that 18s are faster. However, I asked tire guru Paul at Radial Tire (races Grand Am) and Dave Empringham (has raced almost everything) about this and they both said there should be little to no difference in lap times. I also started a thread on the Corvette forum on this for some of their opinions and they said about the same thing with the nod to 17s for gearing reasons.

I will be staying w/ 17s unless I upgrade my brakes and need to change to 18s for that.

David K.

I ran the 225/45/17 - 275/40/17 & 245/35/18 - 285/30/18 Hoosiers at The Glen last week on my RSA, the 18” are faster. The 18’s have better grip, I like the feel better during transitions and turn-in, due to the stiffer sidewalls. The 285/30/18 dia is smaller by 2-3% vs. 275/40/17 which helps the gearing. Also I was only running 225/17 in the front and the braking was not as good as the 245, this would be true for the 17 or 18 as the 245 comes in both sizes.

mark kibort

sure the wheel would weigh more, all things being equal, but since the tire is a greater percentage of the weight, its actually the opposite. you get less wheel material, but now need more tire material. in the end, its about a wash. my fronts 18 or 17 weigh the same. overall diameter is the same too, as you generally use the same overall diameter tire when going from 17 to 18"..

anyway, you are shaving hairs here. SO many other factors that are much more important. Like, condition, and type/ brand of the tires!!!

MK

Bull

Simple, with the same conditions being equal (brand, age, tread, etc) the 18s will have a lower section height than the 17s when maintaining the same rolling radius, giving a lower/stiffer sidewall and more crisp, precise reactions and handling. However, many drivers are not to a point with their driving where they can tell the difference and think it "doesn't matter". If you are on 18s and your friends kick your butt on 17s (all else being equal), pray they don't go to 18s!


I like 17s for the street (the big sidewall is part of the suspension), and 18s for track. If you aren't trying to maximise your speed/minimize your lap times on the track, it doesn't matter.

jakermc

Consider your tire and wheel selection as part of your suspension system. Everything needs to be tuned together as one system. As stated, 18" wheels will have a lower/stiffer sidewall and this results in a higher spring rate. You have to ask yourself if you want higher spring rates. Does this fit your driving style and current set-up?

I don't have any measurements to say how much higher the rates are with 18" wheels, but on earlier 911s running T-bars this is an easy way to increase spring rates beyond the rates offered by commercially available T-bars. With coil overs perhaps its not such a big deal, but the jump I made from 16" to 18" while running 23/31 t-bars was very noticeable and a welcomed change.