Not so much the diameter as it is the rollout. When there is a discrepancy between the rollout of the tires front and rear, the AWD system will interpret it as a fine amount of slippage because the rear wheels are rotating slower than they should relative to the fronts. I am sure that the Turbo incorporates some type of wheel sensor or maybe even uses the ABS sensor to detect this. Sometimes, the center diff can sense this difference as well. Theoretically, the car should compensate by adjusting the center diff to transfer more power to the rear diff. This could be dangerous in that your car may behave more like a full RWD car instead of the AWD car that it is. Considering that your car is an S and has the additional power, it might behave like a RWD GT2 when you least expect it. That is what could technically happen. With the tires you have planned, the car is technically slipping 1.5 inches for each rotation. This is an excerpt from the Porsche website,

"Power is split between the front and rear wheels by means of a multi-plate viscous coupling. Apart from delivering the optimum torque split, this ingenious device serves as a center differential, automatically compensating for differences in speed between the front and rear axles. The coupling itself consists of an outer casing and a center shaft, both of which have a series of interleaved plates. The space between the plates is completely filled with a high-viscosity silicone fluid.

If the front and rear axles begin to rotate at different speeds, the frictional properties of the silicone fluid cause the drive torque to be directed away from the plates that are rotating more quickly and towards those rotating more slowly. At least 5% of drive torque is always applied to the front wheels, rising to around 35% in normal conditions and as much as 40% in extreme scenarios."

Bill, I think if you must go with that combo, be careful and make sure you calculate the rollout of the front wheels/tires to see if the difference is even greater than 1.5 inches per revolution. Be safe my 6speed brotha!

 

There is much merit to all the points brought up but alot of the specs/tech stuff mentioned is theoretical and not actual. My car had 315/25/19 rear with 235/35/19 fronts which many people were/are running without problems. This was done initially b/c that was the sizes made most commonly and if you wanted 19's then that was as close as you could get in the brand of tire you wanted, i.e. Yoko, Michelin, etc. Nobody crashed and burned or had their AWD/PSM go nutty. When it came time for a change I went with the Pirelli b/c they did make a 305 rear and according to the "theoretical" tables this tire was the closest match to stock height and therefore circumference. Everyone hear mostly poo-pooed the Pirelli vs PS2 and nearly everything else. I, like Dr. Bill do not track my car nor drive it anywhere near 10/10ths and the new tires are an improvement IMO and for my driving. Besides that, the 305 which should "theoretcally" have a 10mm less contact patch were actually wider b/c tire manufacturers don't seem to need rulers. There's no question there are better choices than what Dr Bill has coming but certainly at nowhere near the price. Many of us who have modded have done things that weren't necessarily what was recommended by the experts and even Porsche themselves but isn't that what modding is about? If he has quality spacers on there with the correct bolts not much should be expected to happen. And if it does the worst thing will be some stupid computer thing not a wheel flying off. If that happens he puts his stocks back on and takes it in for service, hoping that PCNA doesn't monitor his activity via this site (they probably do). I'll save them some time, his VIN is ...

 

I agree Red Devil, but it's almost like he's modding backwards by going w/ a smaller setup. You mod to improve, not take away. Unless you are strictly a blinger. Could you imagine going up to 19's and going down to a 275 tire?

 

the rear tires would have a greater circumference and this would be approx. 1.5 inches. if the change in my front tires(circumference) is from a 225-40-18 tire to a 235-35-19 tire wouldn't that also be a greater circumfernce therefore compensating for the increase on the rears and giving me a good ratio front /rear without AWD, ETC ISSUES??

 

Dr. Bill,
if you do as you are saying then deffinately keep you stock wheels because the next buyer may be turned off by the missmatch. What you want to do wil work and you should not have any issues but you are compromising. I also comprimised when I got my used rims. They had 9 inch fronts and I wanted 8.5 fronts. I upszed all 4 tires to get the front to "bead up" properly. I can appreciate a good deal as much as anyone but more often than not you can something wrong twice or right once. You guess which one is cheaper in the long run. You should be fine and most wont be alble to tell when looking at the car. Unless you are saving a ton you will also be buying new tires for this setup. If you are getting the tires also with the rims as a dealconsider if they are the tires you would buy seperately. I have the PS2 and LOVE them. Very good street/ light track tire. Good luck!

 

to steal a wheel/tire set that looks nice and i don't track the car for 20mm less width, absolutely...

 

Well, you seem to be the only one, and since it's your choice, it's all that matters. But has everyone has mentioned, you get what you pay for.


Personally, i'd stick to my recommendation, sell the wheels since you got a steal to someone w/ a narrow body. Then buy yourself the proper setup.

 

DrBill,


you seem to be stuck on the issue of the offset change is not that big of a deal for someone who doesnt have the track or performance in mind, you are right on one aspect, offsets are made to be flexible. There are other factors to consider and pay close attention to. Changing your offset is not a problem as long as it fits, it will still roll your car along(not to the best standards). however, where your tires meet the pavement is the real issue with such changes in an AWD car
check out this link http://www.dunntire.com/WhatYouShouldKnow.aspx?id=2

Maybe this will relate. Should a doctor prescribe someone a drug who is healthy and doesn't call for the drug all while knowing the drug could potentially then make the person develop conditions that were non-existing before prescribing?

 

Doc, the 225/40/18 wheel/tire and the 235/35/19 wheel/tire have the same rollout. That is the perfect "plus 1" upgrade. Same rollout, same overall diameter. A lower profile tire to compensate for the larger rim. You are back to square one. The 235/35/19 doesn't help the situation at all. In fact, it would make things worse because you are increasing traction in the front by increasing the contact patch by 10mm (approx 1/2 in) and decreasing traction in the rear by decreasing the contact patch by 20mm at the same time. That plus the potential AWD/rollout issue, you are upsetting the handling even more than stated before and getting more and more into an oversteer biased situation. Now there are three things working against you. I would call that a strike out.

 

John great link ..if that doesnt explain it best i dont know what will ...

 

Red Devil,

You are right on all accounts but your situation was not the same as Doc's.

235/35/19 = diameter of 25.5 in
315/25/19 = diameter of 25.2 in
305/25/19 = diameter of 25.0 in

The rollout discrepancy of your "incorrect" tire combo was only about 1/2" per revolution of the wheel/tire. His is 3x yours.

Also, you are absolutely correct that each tire manufacturer is different and every tire of the same size actually sits differently on each wheel. That's dependent on tread pattern, tire design, and the way the tire beads up against the rim. Just like tire wear ratings are inconsistent from manufacturer to manufacturer. They are also pretty consistent within the same manufacturer. All the more reason why you should be running the same tire front to rear. The measurements should be relatively consistent within the same tire model of the same manufacturer because the tread design, tire design, and bead should be the same.

 

Just got off the phone with porsche parts rep here in long island and we were conferencing with the Continental rep for the tires. Here's where we stand.....the stock 225-40-18 in the front have a revolution/mile of 825 and the rear 295-30-18 have rev/mile of 827. Porsche specifically contracted with tire company to create as close a ratio to rev/mile front rear in order to preserve the sensitivity of the AWD system of the turbo. Up to 1998 the rev/mile of the turbo had a front/rear of 847/867, resp. The present model turbo and turbo S has a greater sensitized AWD system and and therefore the rev/mile front to rear is much closer. The tires i purchased have a front rev/mile of 819 and the rears of 817. The similar range of the stock and my new tires is exact! The tire diameter of the new front/rear 19's are both 25.5.

In addition, the porsche rep also discussed the change of increase on the front tires contact patch of 10 mm and the decrease of tire patch on the rears of 20mm. The porsche, as well as most high performance vehicles were made to understeer instead of oversteer for obvious safety reasons. The degree of steering change will not be a factor here.

 

Bill, it's obvious you want to take the advice of your tire guy, and not that of those here. ALl we can say now is best of luck, post some pics when you get them on

 

Let me clarify, this was not a tire guy. This was a northeast porsche rep and a continental USA rep....Here is the rev/mile for my 275-30-19's rear and 235-35-19 front.
http://www.tirerack.com/tires/Spec.j...el=Exclaim+UHP

 

Best of luck bill, still don't see why you'd upgrade and go w/ a smaller tire. THat was my point. I think it's obvious they will fit, but I bet even the tire rack guys were a little puzzled as to why you were downsizing.

Enjoy the deal you got on wheels, and keep us updated when you get pics