Originally Posted by Doug_B_928

I've since done some further reading. It seems that the wider rear tire might make for a bit more straight line grip but in the corners the change in width ratio front to back will produce more understeer (push). Though, for most this will likely be imperceptible.

What I'm still left wondering about is the larger diameter of the 305 vs. the 295. There is not much difference, but I would like to know if that would potentially have any negative effects on the car.

Originally Posted by 911cv

I don't think Porsche (or Tirerack) would sell 305s for a NB 997 if it had a negative effect on the car.

When I was making my decision, I used this website's tire calculator (it was helpful in comparing the 295s to the 305s): http://www.1010tires.com/TireSizeCalculator.asp

I've had mine for about 6k miles now and have not noticed any difference in handling. The visual difference is there if you have an eye for detail.

Originally Posted by 997S.Aero

I have Mich SS 305 30 19's with 5mm spacers on a C2S and the edge of the tire matches perfectly with the fenders's outer edge. Also added 5mm's to the front. All fit perfectly and greatly enhances the aggressive stance.

Originally Posted by LPpfs997

The 305's have been standard for 19" c4 and c4s for five years but these 997 pcars are 1.5" wider then the c and cs. The driving dynamics as well as unsprung weight will be affected : from Wikipedia. I'd speak with Damon at TireRack before proceeding.

Unsprung weight


Ignoring the flexing of other components, a car can be modeled as the sprung weight, carried by the springs, carried by the unsprung weight, carried by the tires, carried by the road. Unsprung weight is more properly regarded as a mass which has its own inherent inertia separate from the rest of the vehicle. When a wheel is pushed upwards by a bump in the road, the inertia of the wheel will cause it to be carried further upward above the height of the bump. If the force of the push is sufficiently large, the inertia of the wheel will cause the tire to completely lift off the road surface resulting in a loss of traction and control. Similarly when crossing into a sudden ground depression, the inertia of the wheel slows the rate at which it descends. If the wheel inertia is large enough, the wheel may be temporarily separated from the road surface before it has descended back into contact with the road surface.
This unsprung weight is cushioned from uneven road surfaces only by the compressive resilience of the tire (and wire wheels if fitted), and which aids the wheel in remaining in contact with the road surface when the wheel inertia prevents close-following of the ground surface. However, the compressive resilience of the tire results in rolling resistance which requires additional kinetic energy to overcome, and the rolling resistance is expended in the tire as heat due to the flexing of the rubber and steel bands in the sidewalls of the tires. To reduce rolling resistance for improved fuel economy and to avoid overheating and failure of tires at high speed, tires are designed to have limited internal damping.
So the "wheel bounce" due to wheel inertia, or resonant motion of the unsprung weight moving up and down on the springiness of the tire, is only poorly damped, mainly by the dampers or shock absorbers of the suspension. For these reasons, high unsprung weight reduces road holding and increases unpredictable changes in direction on rough surfaces (as well as degrading ride comfort and increasing mechanical loads).
This unsprung weight includes the wheels and tires, usually the brakes, plus some percentage of the suspension, depending on how much of the suspension moves with the body and how much with the wheels; for instance a solid axle is completely unsprung. The main factors that improve unsprung weight are a sprung differential (as opposed to live axle) and inboard brakes. (The De Dion tube suspension operates much as a live axle does, but represents an improvement because the diff is mounted to the body, thereby reducing the unsprung weight.) Aluminum wheels also help. Magnesium alloy wheels are even lighter but corrode easily.
Since only the brakes on the driving wheels can easily be inboard, the Citroën 2CV had inertial dampers on its rear wheel hubs to damp only wheel bounce.

Originally Posted by Doug_B_928

Hmmm.....interesting...... I was at the dealer yesterday and they had a C2 cab with 305s and the tires stuck out past the wheel well a bit (maybe .25"). I think they were Brigestone's. The wheels had also been switched with another 911. Probably the switch in wheels is responsible??

Originally Posted by 911cv

I have the same setup -- 305s with 5mm spacers. Because the wheel well is flared, the edge of the tire is not flush with the entire wheel well.