dunkd

I am looking at a set of 996 Sport Technos that have 90% Pilot SS tires, it's a good deal but I am concerned about the tire sizes that are mounted on the wheels.

Question is, what is the reasonable diameter tolerance between front & rears on an AWD car?

The wheels have 225/40/18 fronts and 285/35/18 on rear.

Specs per http://tire-size-conversion.com/tire-size-calculator/

Tire: 225/40/18 /// 285/35/18
Dia: 25.1 /// 25.9
Wid: 8.86 /// 11.22
Sidewall: 3.54 /// 3.93
Circ: 78.8 // 81.2
Revs/Mile: 804.3 /// 780.5

OEM spec calls for 295/30 in rear which are:

Dia: 25
Wid: 11.61
Sidewall: 3.48
Circ: 78.4
Revs/Mile: 808.2

2K7TTMIA

on 1010tires.com website, the 285/35 vs the 295/30 shows a 3.55% difference in diameter variance, which is outside of the "allowable" tolerance. A 996/997 C2 could probably get away with these sizes; I'm not sure if its a good idea with C4's/Turbo's....
I havent seen any recent posts where the 285/35 rears are ok on AWD Porsches.

YMMV

dunkd

What is the allowable tolerance? Thanks for the help.

2K7TTMIA

The internet says "as close to 0-3%". On 1010tires.com, the 225/40 295/30 combination yields a -0.47% variance; meaning, the rears are marginally smaller in diameter ( as you showed in your first post). Porsche OEM specs for 8" front and 11" rear wide wheels.

I plugged in 225/45/18 and 285/35/18's and the variance was -0.45%, which is real close to the OEM variance.
Since you mentioned the MPSS's are at 90% remaining tread, it's up to you if you are willing to purchase the current wheels/tires and end up changing the fronts or rears. ( less expensive for fronts).

Dennis C

285/35/18 is the wrong size for the C4S. You should ditch those tires and buy the proper sizes.

zabuch

Hey denis c by any chance do you kno the firing order on a 3.6 I am getting a misfire on #5 trying to find out witch one is #5

fpb111

1,2,3 is under the expansion tank. 4,5,6 is under the power steering res. #1 is in front of your left knee when you are gazing into the open engine bay. #4 is by your right knee.

#5 is middle of right side as you look in.

Peter_S03

My impression was : 225/40/18 & 285/30/18 , is what I have on my 2001 C4, was this the wrong size ?

Seeeu911

For the 11" rim on the wide body rears 295/30 is what you need.

944T4ME

Sometimes I really don't understand Porsche people. There is no "proper" size. I am not usually well-liked on forums as I don't follow the herd. I do interesting projects and projects that most people wouldn't do. I ask questions that a lot of people respond to who only know what they've read on forums. When I ask why something is so, the answer is not usually satisfactory.

Any size is ok for an AWD car as long as the tire sizes are the same height, or close. Close is .2" or 10 revolutions per mile. Just know that your handling characteristics will be affected depending on the size tires you use. You can use this to your advantage by dialing in the car to your liking. You can dial in understeer or oversteer to YOUR liking, not Porsche's or anyone else's.

People keep using these online converters. Try using maths for once. It is simple and you will learn more about tires.

To figure out the height of a tire, first figure out the sidewall height. The sidewall height is given as a percentage. This percentage is the percentage of the tire width, which is given in millimeters. For example, the sidewall width of a 295/30/18 is 88.5 millimeters or 8.85 centimeters. Times 2, since there are 2 sidewalls, is 17.7. Divide 2.54 into it and we have 6.7". Add the rim size of 18" and we have 24.97". Doing the same for 225/40/18 is 25.09". This is a difference of .12" Which is great.

285/30/18 is what the owner's manual says which is 24.73", or a difference of .36". Too much, but that's what Porsche says. What will happen? No idea, I don't know enough about C4s. But since it is in the owners manual and there aren't thousands of blown diffs around, probably nothing.

The OP's size of 285/35/18 is 25.85", a difference of .76" and is way off for an AWD car.

I personally have a set of 245/40/18 (25.72") and 275/35/18, (25.58") which is a difference of .14" which is great for an AWD car. How would a C4 handle with these on? I don't know, why not put them on and try it out!

The point is, any size is fine, just as long as they are close in height or rolling length (remember pi?) so you aren't constantly spinning the diffs. As far as how the car handles, who knows but you probably won't find the answer here. But if you already have a set or can get them cheap, throw em on and see.

jims911

Your C4 was a narrow body, the C4S is a wide body, and takes a wider tire (like the 295).

Dennis C

Which part do you not understand? Porsche recommends one tire size for the C4S rear tires. They only sold the car with one rear tire size. That's the proper size.

Obviously a C4S owner can put any tire on their car that they choose. Like any choice you make with respect to your car, there will be consequences. Sometimes consequences are good, sometimes they are bad. If you buy the proper size, then you don't have to experiment and make guesses about whether things will be okay or not.

That's all that "proper size" means. I think you might be reading too much into it.

Dennis C

Thinking about this a little more, I'll add a few comments. I don't think that the proper or recommended size is the only size that will work. I know several people who run 305 width tires on a 996 C4S who have no issues. I know people who run 19" wheels with no issues.

I mentioned running the proper size because it's a size that we know works. It's a size you can run without guessing. I don't think there is anything wrong or bad with using the sizes that Porsche recommends.

onewhippedpuppy

Going with the Porsche sizes is easy - you know they will fit. Further, you know that the driveline and suspension was optimized for that size of contact patch, approximate weight, diameter, etc. While I have zero issues with experimenting, that can get expensive fast. A quality set of 18" tires is $600+, that's a lot to waste if you find that they rub the fender or liner. If you have the extra cash to try out different tire sizes, by all means do so and post your results.

996_C4S

I'm with Dennis. Esp for those like me who've done suspension mods like lowering springs. There are other parts of the car (e.g., most parts) where experimentation can be done with more confidence.