"02996ttx50

that's within the factory recommended spec ( and your sizes are good for 19's,.. i ran 'em 'til i learned better ) of less than 3% sizing differential front to rear so you're ok and long term issues that might arise from a lousy front to rear pairing shouldn't result... except your car has 19's on it. why? we will never know. somebody lied

if you want your car to run as it was designed, you'd be best served by going back to 18's. the car handles best with either a 8.5 or 9 in. front, and a 12 in. rear, ( and lowered and optimally with suspension help.. next issue..) that said, if you do return to stock sizing you can however run a 315/30/18 on the stock 18in twist without issues for the widest possible stance. it's a decent patch of rubber. also, barring a 12 rear, the stock 11 is fine too. not best. but you have 19's. so for now, a moot point.

it will never matter if all ya do is drive around town. but, if you do get serious about your driving conditions..? go back to 18's. in the meantime, align your car for the taller wheels, and wear out the old tires and do what makes ya happy. congrats on your pickup! best old sports car around

"02996ttx50

why did they tell you that?

l8tebr8ke

The wheels were changed by the previous owner. I was initially planning on changing the wheels back to the stock 18s but the look of the 19s are growing on me. If the diameter difference doesn't cause a problem them maybe I will run them until the tires need to be changed. I do plan on tracking it down the road and that might push me to go back to the stock wheels. In any case I am loving this ride!!!

adam_

Those sizes are wrong...DO NOT listen to anyone telling you they are within spec.

On what planet does a 0.7 inch difference equal 3%??????

Revs per mile on thos sizes are 817 versus 788. That is a WHOPPING difference of 29,

Stock from the factory the delta is THREE (829 & 832) 3/829 is 0.4%

29/788 is 3.7%.... BUT THERE IS NO 3% SPEC..this is an internet rumor.

Those tire sizes will cause ~10 times more heat and wear on the differential than stock.

If people tell you otherwise, ask if they will pay for repairs.

"02996ttx50

i ran those sizes at the recomendation of tirerack and didn't have issues beyond i didn't like the way 19 in wheels felt on the car. if that's not within the 3% spec, i stand corrected. i don't have the charts i used for comparison and no longer concern myself with inequal tire sizes. if i'm not mistaken, those referenced sizes are common on 19 in setups. no?? mine were like 2% differential but i'd have to have the numbers again.

can't find my old 19in spec sheet to compare since you said i was nuts. but this is all i concern myself with now re tire sizes. 18's.

Spec Sidewall Radius Diameter Circum Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
235/40-18 3.7in 12.7in 25.4in 79.8in 794 1.3%
295/30-18 3.5in 12.5in 25.0in 78.4in 808 0.0%
315/30-18 3.7in 12.7in 25.4in 79.9in 793 1.9%

the 315 has a diameter of 25.5". The 295-30 is 25.1"

the 235- has 25.4", and the 225-40 is 25.1"

"02996ttx50

you may find if you lower your car an inch or so with 18 in wheels, much of the look you like from the 19's will be retained, while the car will handle better.

SSST

Between these 4 sizes The 225s match up with 295s (standard 996 TT size) and the 235s match up with the 315s (standard 996 GT2 sizes). Either set will work on the TT.

If that's want you are saying then you are on the right track.

"02996ttx50

i was totally off base with the tire sizing. adam was correct, the differential in the op's tire sizes in nearly 7% and that ain't right. not sure what i was thinking, and i never ran more than a 2.5 difference front to rear. math isn't my strong suit, but drinkin' is. i'll stick to that for now..mea culpa.

"02996ttx50

yes, those are totally within spec and optimal. i'm not sure what i did with my math calculator, but again.. blaming it on simple enough, and we've all done it.

l8tebr8ke

Thanks for all the info. As I said earlier I was planning on switching back to the stock 18s. This will definitely influence my decision. If I decide to keep the 997 19s, are there certain tire sizes I should consider?

l8tebr8ke

I also noticed that the car doesn't feel as stable as other TT I've driven. The shop where I did the ppi did say they would be less stable at speed. Can this be corrected with the right setup. Or should I just go back to the 18s?

"02996ttx50

exactly.

SSST

I have come to the conclusion that 19's on the 996 TT are more about looks than performance. The car was engineered for optimum performance with the 18" sizes. If you want to improve the looks, lower it about an inch and put 7mm spacers on it. This will also improve performance if done correctly.

powdrhound

Assuming you guys understand how the AWD system with a viscous coupler works on the 996tt, you must realize that running a 3% larger rear than front is a TOTALLY different proposition than running a 3% larger front than rear. The 996tt AWD system is a bone simple mechanical system that relies on a difference in the rate of rotation front to rear to transfer torque. The rear tires must rotate faster than the fronts in order for torque transfer to the front to occur.