the problem with tire sizes is that diff will see different sizes than expected,(if you change to 235/295)and so it will think wheels are spinning so feedback is needed...by doing so permanent it will overheated and finally burned...Mike,check your front diff if it operates properly...(lets hope it does)

 

I've swapped back and forth over the 3 years I've owned this car, almost from the beginning, keeping within the 3% variance without issues. Car is in the shop now getting the trans rebuilt and they checked the front diff... It's fine.

Mike

 

According to the Michelin site, the diameters of their tires are as follows:

225/40-18: 25.1"
235/40-18: 25.4"
295/30-18: 25.2"
315/30-18: 25.5"

So using the 235-295 pairing gives a difference of 0.2" instead of 0.1" in diameter. You could get that variance just from different amounts of wear front and rear.

Is that the spec, Mike, less than 3% difference in diameters? That would be almost 3/4" difference! That would allow you to use a 245/40-18 with either the 295 or 315 rear.

 

Guys if this is the case next time i will put 235 front...Has anyone else tried it?Mike i am quite happy you use them without any diff issues...Maybe i can eliminate my understeer problem now...

 

Second question to Mike is: Did you put the 235's on an 8" wheel? Michelin calls for an 8.5" wheel for the 235.

 

I can't be 100% sure on the 996tt, but I know most AWD manufacturers put strict tire requirements for insurance on their part for new cars. A .2 difference is not very much at all. Tire wear can easily cause this. Some tire manufacturers have varying tolerances on tire rotational diameters that can cause a .2 difference. Correct me if I am wrong, but doesn't the 996tt use a viscous clutch pack on the front differential to distribute the torque ratio from front to rear. If that is the case, every time you turn the car the clutch packs are wearing the exact way they would be with missmatched tire sizes. That may also mean that you could be inducing more diffential wear tracking a car than with tire diameter differences. I could be wrong on this, but I am not buying that the front diff is as sensitive as it is rumored to be. I would bet that blown front diffs were due to numerous things besides just tire sizes.

 

Sorry guys, was away from the computer all day...

Jon, I thought the variance in diameter quoted was 3%, but I may be wrong. I'll have to go back and check my notes. Bottom line is the 235 was within the tolerance/range for diameter differences. Remember, there has to be a minimal amount of difference front to rear or you'd have issues with heat related pressure expansion. Tires grow under pressure. How much? I don't know, but they do expand in all directions when at max pressure and max heat.

I used the stock OEM Turbo wheels without any issue. That's an 8inch wheel... and BTW, The guys at tirerack are with me on this one... There are ZERO issues to my knowledge with running a 295 rear and 235 upfront. Also, Michelin calls for a RIM WIDTH RANGE of 8-9.5 inches. I ran it on the 8inch rim with ZERO issues for the two years I ran this combo and will likely run it again this year.

Mike

 

I couldn't find it in my owner's or shop manuals.

Sounds like I will as well!

 

why doesnt someone just call porsche?

 

Jumping from a 95-993 with an RS suspension to the 996TT was a big step forward in power but a major step backwards in handling. There are lot of causes (SUV ride height, all the extra weight, and the AWD). You can cure the SUV ride height and compensate for some oversteer with adjustable sway bars. Unless you get a full on GT2 suspension with LSD and eliminate the AWD, I don't think you'll ever get near the playful handling of a N/A 993. On the 996tt i put PSS10's and adjustable sway bars, replaced some of the "soft" suspension parts in the rear end (upper control arms, toe steer link, drop links and motor/tranny mounts). This helped a great deal but even with the PSM off, as you get near the limits the AWD likes to take over a create understeer. I would be careful with the tire/wheel changes to stay within the AWD parameters (assuming you keep the AWD). I have a set of 19"s with 235/315 and a set of stock 18"s. The 19"s provide more of bling factor but in all honesty the 18"s handle better.

You can go with the trial and error approach, but you're better off finding a good shop with lots of experience with these cars and let them set it up right the first time.

 

I have no idea what pp have said so far.
But you DO NOT need anything fancy to reduce understeer.

All you need is an alingment.

Get some toe out the front and toe in at the rear
get a good negative alingment. I'll find my spec sheet
and BAM, i can throttle steer/drift the car like a gokart,
with very little understeer...... toe in at the rear and out at the front
is not for the faint heart.

 

Toe out at front?for sritited road use...interesting...i use +1,0mm total toe in front and +3,0mm total toe in rear and the rear end is glued on the road...Also on Tuesday I set pss10 one point harder in front...and the car was very different...Understeer almost gone...Front end seems way much stiffer than before and turn in is better now...I like it better that way...

 

Toe out in front is good for the track but I personally don't like the front end wandering on the street. A little toe in at the front is more neutral. Maybe there is more understeer, but with all the other suspension upgrades I have, I don't notice it. To each his own.

 

I see...so toe out in front causes car to hang left and right at straight line...Every time i have this feeling i check allignment and always find front toe out...Weird...i reset it to +1,0mm toe in and after 3000-4000km again i have the same feeling...

 

There is nothing wrong with that, it's under 1% (.007) in variation and will not cause any problems, ever. All tires can't be grouped together as heights at different profiles will be different from one tire to the next. You have to research the indvidual tires, but the numbers given before was a good guideline for where to start.


I think the rear sway will reduce the understeer plenty enough for the street. I've never had turn-in oversteer in my car unless my steering inputs were not smooth and even then it was easily correctable.