MCharbe

Rennlister,

I bought 18inch wheels for my 951S - 18x8et50 front and 18x10et65 rear - and I am now looking for tires. Based on my online search, I might be going with 225/40ZR18 and 265/35ZR18.

Before buying, I'd like to have comments and recommendations regarding tire size for these wheels. So please feel free to post any!

Regards,
Mathieu

Dougs951S

I'm running 996 N/A twists, 7.5x18F 10x18R. current sizes are 235/40/18 and 265/35/18. My next set will be 235/40/18 and 295/30/18. An added benefit (to me) is that the 295/30 out back will give you SLIGHTLY taller gearing (around 1/2% taller than stock tires) which is a good thing if you are pushing decent power at the wheels.

admiralkhole

225 on an x8 and 265 on a x10 wheel will stretch the tire. It's definitely possible, but there's a lot of room if you want to go wider. I've also heard you want to stay within 30mm apart between front and rear, I've had good results from doing so. You can try 245 and 275 respectively, 275 will even have a slight stretch on a x10 wheel.

odurandina

I run 8 1/2" wheels up front with 255-35-18s

and 10" wheels in the back with 285-35s.

catamount

If you can find tires in 285/30-18 you won't have any issues with the speedometer. Common conversion sizes are 255/35-18 and 265/35-18 but you'll be running about 1.5-2 mph faster than your speedometer.

on my '87 951, I'm running 225s up front and 255/35-18s on the rear because those were the only sizes I could find in a matching set last year. (7.5x18 and 10x18 Sport Classic II wheels). The rears are stretch a bit but fit okay. I'd like a bit more tire back there though and there's room. The front has enough for an 8" wheel but going too much wider would create rubbing issues.

morghen

I'm running 225/45/17 on all four wheels, all four are 9" wide.

I used to run 225 on fronts(7.5" rim) and 245 on the rears(8.5" rim), but with 225 all around the car feels perfectly balanced while with that diffent tires setup for front/rear it did not feel and drive like that.

Contact patch is important, but if you're not really racing, big tires will only cost you much more money with no real benefit.
First, the wider tires cost more to purchase, second your fuel efficiency will drop, the car will feel heavyer to move around and the power steering will have to do more work.

My opinion is that the performance of the specific tyre you are using is VERY important and you dont have to use fat tires to have enough grip. I'm using Conti Sport 5, the car is ON RAILS at any speed. Its even decent to drive in the city and in the last 3000Kms it averaged 27mpg, holiday sporty driving (highway,mountaim roads and a bit of city driving)
When i was running 225 on the fronts and 245 on the rears with inferior performance tires i did not have so much grip as i do now with premium top performance Conti Sport 5s.

pics with the setup:



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gregeast

225/40/18 and 265/35/18 on Turbo Twist II's, 8 inch front, 10 inch rear.

There's room to go wider in back but I like the feel of the steering on 225's in the front and didn't want to get too big a disparity between front and rear.

To answer an unasked question, I run 36 psi all the way around. I asked on the 996 board (since these wheels were OE 996) and their door jam says 36 front, 41 rear. Since our cars don't has quite as much, ahem, "junk in the trunk" I decided to go 36 at all four corners. YMMV.

odurandina

this absolutely would not be true for a 968.

and I doubt for a 944 either.

a 25" tire (265 or 275-30-18) will give a significant false speed reading after 70 mph.

for example, at 140mph, a 968 would show an indicated speed of 150~153mph running tires with a '30' aspect ratio. you need a 25.6" diameter tire to achieve the correct 25.0 mph/1000 rpm.

or a tire yielding approximately 813~818 revolutions/mile.

a significant speed defect on virtually all Porsche speedometers up to their most recent cars is well documented.

JJR512

What exact wheels do you have, and what are their offsets? Any clearance issues at all, ever, under any circumstances? Are there any wheel/tire fitment differences between a 968 and 944 (i.e., if they fit on your 968, will they fit on any 944 with no problems as well)?

MCharbe

Thanks all for your comments! I think it's gonna be either 225/265 or 235/275. Is a stretched tire on a wheel only a cosmetic thing or it can create performance issues? (Looking on the tire manufacturer website, the rim widths are good for both sets.)

Pictures showing tire fitment on wheels for combinations of tire and wheel sizes would be nice too!

Erzengal

Proper sizing is 225/40 front, and 265/35 or 285/30 rear. Until some of the newer Porsche's, their wheel and tire sizing is pretty common.

catamount

The 968 base 16" tires were the same size as the 951, 205/55 and 225/50. So, the model really shouldn't matter.

A 225/50-16 is running 811 RPM at 65 MPH.
A 255/35-18 is running 806 RPM which would be 65.4 MPH on the speedo
A 265/35-18 is running 797 RPMs which would be 66.2 MPH
A 275/35-18 is running 789 RPMs which would be 66.9 MPH
A 285/30-18 is running 816 RPMs which would be 64.7 MPH

So, the 265/35 and 275/35 sizes will cause the speedometer to be significantly off at freeway speeds, which is what we both are saying. The closest 18" options are the 255/35 and 285/30.

As for the speedometer accuracy of the 944, well let's face it, someone at Porsche (probably the lone Frenchman that works for Porsche) must think the speedometer should give a best guess rather than be accurate.

odurandina

the factory may have put a 215mm front tire on a 944 long ago.

the factory may have put a 215mm front tire on a 968 long ago....

that was how they did it. we can speculate as to why. my random;

1. cars mostly didn't come with big fat front tires back in those days.
2. tires were put on the cars to get them down the road. not be the closest thing to F1.
3. the 944 was sold as an entry level sports car not a flagship car or supercar.
4. Euro cars ran really skinny wheels back in the day.... the creep forward was slow.
5. when you put bigger tires on, the fuel mileage suffers. gasoline costs were a huge concern for a long time. car manufacturers for the most part, ain't about that.
6. shhhh... the 944 and 968 weren't supposed to corner better than the 911. but **** if they didn't anyway, running those crappy, narrow tires.


anyone who thinks a 944 or 968 is optimized by running a 215~225mm front tire, and then a 265~295mm rear is mistaken.

the idea that a 944 or 968 is optimized by running a significant tire stagger is mistaken.

it just isn't true. it might look kind of cool. whatever.

but, we're conditioned from the sports car and 911 world.

our cars are actually at their best running minimum to no stagger at all.

run a small front tire and a big rear. the car will push. at the limit, your car will keep going straight.

I run a 255 in the front and a 285 in the rear. tire profile is perfect on the 8.5" rim, the ride is fine, and the car corners like it's on rails.

want to go small (215~235) up front? keep the rear tire maximum width at about 245~265.

want to run a big rear tire (275~285)? then, run a big front tire (245~265).


the guy going 155mph at Fontana in his 944 runs the same width front and rear tires. why?

because he prefers destroying his competitors instead of losing.


and this poor bloke?... umm, yeah he thought running a big stagger like his 911 buddies was cool....

may work ok on a 911, but don't try this at home....

ain't pretty.

http://www.youtube.com/watch?v=2SNqI...yer_detailpage





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JJR512

All the speeds you've given are within 2mph of the target. I wouldn't consider that to be "significantly off". I'm not sure about modern-era cars, but it used to be not uncommon for factory speedometers to be off by as much as 5mph at 60mph even when running original stock tires. This 5mph variance used to be well-documented in the various magazines of the time. And it's also why it's extremely rare to be stopped by police for speeding if your actual speed is within 5mph of the limit.

The variance is also why police patrol cars have calibrated speedometers. If speedometers were inherently accurate, there wouldn't be a need for that.

odurandina

no. at least for a 968, a 275-35-18 is a 25.55~65" tire -and is absolutely perfect.

Porsche engineers intended/designed the 968 transaxle to make 25.0 mph/1000 rpm in top gear.


many of you run slightly smaller tires (between 24.8" and 25.1").


on my 968,

I turned my speedo back about 4 mph. then ran the Dunlop 275-35-18/25.6" y tires.

when the tires are new, my speedo reads 100, I'm going 100.0.

when my speedo reads 150, I'm going about 149.8...

the defect is virtually nil.

200 miles of mile markers and my odometer is reading 199.9 miles.

200 miles of mile markers and my gps is reading the same.

then you re-calculate using math: diameter of the tire, gear ratios and it comes out exactly as it should.

do the same again with rotations per mile and it comes out the same, again.

25.0 mph/1000 rpm in top gear.

speed at the redline? 162.5 mph. perfect.


how does a 35 aspect ratio tire ride? less harsh than 30 a.r. tires.

how does it look? fantastic.


now, if I choose say, a 285-30-18" P-Zero, or Dunlop and don't set the speedo pointer back.... when my speedo reaches 160mph, my actual speed will be 150~151mph.

on a 968, the short 25.0" tire will be causing a 5 mph defect, and the speedo will be causing the rest.

I prefer 285mm over 275mm

but, when the 285-35-18 tires are new, they're a little too big.

so I go for slightly used with 2mm of wear.



if I were driving a 944T, I would want to be on 25.6" tires.

if running a n/a motor on it's second 100k miles, I'd go with 25" tires...

it would make the rpms go up a little, but it would liven up the car.


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