Luis de Prat

A guy from another forum has a 1986 944 NA with 15" phone dials. When he bought the car, it had 195 65 15 sized tires on it. He later realized that the factory size was 215 60 15 and put on a new set of tires in this size.

He says the car now feels more sluggish and his fuel mileage is down with these wider tires. Since I've never used smaller sized tires on my cars, I wonder if the handling gain (as in preventing understeer/oversteer) is enough to give up the advantages of a more narrow tire, if any.

Anybody?

ERAU-944

have him make sure they're properly inflated. either that or they screwed up something else on his car at the shop. poor fuel mileage you say, make sure they didn't puncture his fuel tank... D'OH

i get upwards of the mid 20s when i'm light on the go pedal.

xsboost90

my 944 had a set of 17" boxter wheels with 255's in the rear. My car did seem alot peppier with the 15" 225/50/15 race tires on the factory rims all around. I imagine you would have more friction due to more contact with the road causing a loss in milage, not drastic though. I do know that these cars like to have the same size all around.

Dave

"Better" is subjective in some cases, different sizes are good for different things.
First, keep in mind that the contact area with the road is a function of tire pressure, not tire size, tire size controls the shape of the contact area but pressure controls the size. A wider tire gives a wide but short contact area (think horizontal rectangle) while a narrow tire gives a longer but thinner area (vertical rectangle). Both shapes have pro's and con's.
The big advantage of a wide tire is lateral grip, we're talking about Porsches after all. I'm not sure which shape is prefered for braking but I suspect the wider tire has an advantage when it come to stopping, also.
A narrow tire is less suited to the type of driving that one thinks of when talking about Porsches but for every day driving a narrow tire has some advantages. A narrower tire has less rolling resistance (think bicycle tire) which would explain your friend's experience. A narrow tire is much better in snow or rain (skis, skates) since they have to displace less of it as they move forward, my snow tires are 195/70-15 for a reason! A narrow tire is more resistant to following grooves in pavement FWIH (and in my experience) but I've never heard a good explanation for this.
Oversteer and understeer can be adjusted a little by changing the width at the front (or rear) compared to the rear (or front), going wider at both ends doesn't change anything. That said, springs, sway bars or shocks are better ways to adjust your handling, tires can only fine tune it a little. To really answer your question, on the track, wider is better is a good generalization but for a car that is primarily a daily driver, a wider tire than stock will have more drawbacks than it's worth (unless vanity is high on your list). Porsche did pretty well when they designated the stock sizes, for highway use the stock sizes are a good compromise and the factory has to realize that 99% of the cars they build will probably spend 99% of the time on a highway.

Geo

I am not suprised at all by his comments.

First, the narrower tire is going to be easier to turn with the steering wheel. Most people are going to equate that with better handling whether it's the truth or not. You see, what we feel in the car and through the controls is how most people judge the car. If the controls are lighter they are going to feel the car is more responsive (keep in mind, light controls and lack of feel are separate things). The wider tires will feel more sluggish because it will take more effort to drive the car.

The narrower tires will also give less rolling resistance so I can see where his mileage would go up, but as correctly pointed out, inflation plays a major role (probably bigger one at that).

Now, something you didn't ask about, but is germane to the discussion is that yes, it's possible to have too much tire. Racers of smaller and under powered cars know this only too well. Too much tire can actually slow a car down through the corners and really make it bog on the street. I don't think this would be a problem on any of the 944s (although 275 tires on an NA probably would), but it certainly is on many cars.

Yabo

Smaller tire also could make it feel quicker.... Smaller tire=faster acceleration but less traction and lower top end...

Manning

Devon,

I think we are talking about width here, not diameter. But to your point, smaller diameter tires will indeed accelerate more quickly.

Yabo

Oh, sorry about that, I misunderstood.

Serge944

My pirelli 205's are so easy to lock up in front. Tires DEFINITELY affect braking. Good thing i have a set of sp8000's coming soon which are wider. Even my good year 215's were wider than the pirelli 225 rears...

Dave

Nobody said they didn't, the question is; is a short, fat contact area better than a long, narrow contact area when it comes to braking. Since you didn't mention which Pirellis you have, I can't even guess weather the new tires will help, the difference in size probably won't matter.

Serge944

I have zr p7000 supersport, which grip very well laterally. Better than my good year eagle gt2's.... Yet it took a lot more effort to lock up the good years. The pirellis are a good 2 inches narrower.

Luis de Prat

Serge944, I think Dave was asking the tire size, to compare braking for each tire.

Serge944

oh, the good years were 215/60/15 while the pirellis are 205/55/16

Dave

No, I was wondering about model, an old set of P3s would be a big difference but his P7000-SS should not be much different than the sp8000, both are a bit better than the Goodyears IMHO. I SHOULD have asked for size also but I misread his post and was thinking 205 vs 215, I guess I'm just psychic (or was that psychotic ) I guess well see what he thinks of the 2 different but similar quality tires in slightly different sizes once he tries them out. I expect the difference to be small but in favor of the wider tire.
As far as a 2 inch difference between the Goodyears and the Pirellis, I'm now just confused. The Pirellis are 205, were the Goodyears 265? A 215 is only ~3/8" wider than a 205

Peckster

Many people buy tires more for looks than performance imo. But you'll have a hard time convincing them their fat tires don't make their car handle better. These things are so subjective, unless you have a professional or very experienced driver who can test different tires on the same car and compare lap times.

I also have to wonder about the current mania for giant rim sizes. Looks or performance? I suspect the former is the biggest factor.