Greetings

I've registered for my first DE event and I need to replace my front tires due to some dry rot. The rears are only a year old, so I'd prefer to replace the fronts only. BUT, I have a C4S and the previous owner put on 19" wheels from a 997.

The current setup I have is 235/35/19 on the front and 295/30/19 on the rear. This puts the rears about 1.9% larger than the fronts. When I inflate the rears higher than the front, I can start to feel the AWD system dragging. So I'd like to take this opportunity to change up the size of the front tires to get a better match.

My options are to go to :

245/35/19 and get circumference down to 0.8% variation

or

255/35/19 and get it 0.0% variation

Does anyone have an opinion or experience with either size on the front? I've seen some people say they like 245. Is 255 getting too wide? My front wheels are 8Jx19H2 ET 57

Thank you!
Charlie

 

The 996 AWD system only works when the front tires are larger in circumference than the rears. This is due to the difference in the final drive ratios between the front and rear.

AFAIK, there is no 19 inch tire combination that will result in the right ratio between front and rear to engage the AWD system.

I now proclaim the heated discussion of the 996 AWD system open for business.

 

I have heard people say if anything you want slightly smaller circumference for the rear wheels - however the manual states it should be equal or within (I believe) 3%. For the record, these are wheels from a 997 C4S, and I'm currently using the same size tires that they come with from the factory. However someone has mentioned that a drive ratio in the AWD system changed slightly between the 996 and 997, hence the issue that I'm having.

That being said - does anyone have experience or a strong opinion on going with 245, or even 255 on the front?

 

Yes. The final drive ratios changed.

 

No you don't, it's all in your head.

The all wheel drive system on the 996 uses a viscous coupling and you definitely won't feel a difference even with 5% difference in wheel size. That said, I'd stick with 235s in the front.

 

Thanks for the advice. You may be right, but I'm pretty convinced I felt it right away after I replaced my rears + adjusted tire pressure to the recommended amounts for the 997 wheels. I can't remember the exact numbers but it was something like 42 in the rear and 36 in the front. I believe that the circumference difference, combined with the inflation variation put it outside of the acceptable spec. When I dropped the rears down to the same as the front, the drag went away - so I'm really keen on getting a different size front so I can go back to the correct inflation.

Quick bump - has anyone used 245 or even 255 in the front? Is there a reason why its a bad idea? Any impact on ride or handling characteristics?

Thanks again to everyone that has responded, I appreciate the insights.

Charlie

 

If I understand the AWD viscous coupling, the greater the difference in speed (RPM) between the front and rear, the more 'engaged' the front drive. The purpose of keeping the circumference of the front and rear close is to minimize overworking the viscous coupling. I typically stay within 2%. with 235-40-18 on the front and 285-35-18 (Michelin 4S) the rear is slightly less than 2% greater in circumference. Somewhere I read the limit is approximately 4%.

Lang G

 

Hey Lang - you're absolutely right. The goal is to keep them the same, and if there is a deviation its best to have the rear smaller than the front. Unfortunately the previous owner of my car didn't do his/her homework and put on 19" wheels from a 997 and my rear tires are larger than my front. Since I need to replace my fronts, my hope is to get slightly larger fronts to match up better with the rears. However, the only way I can see to do this is to get slightly wider fronts (going 245 or 255). I don't know much about tires in terms of how that will impact drivability and handling characteristics so I'm doing a bit of crowd sourcing here to see if anyone has first hand experience with those sizes.

Cheers
Charlie