Tires prove to be a problem!
06-09-2003, 03:51 PM
#1
Burning Brakes
Thread Starter
Tires prove to be a problem!
O bloody hell and my new 951S tires again!
My car is due for new rear tiers. I have Bridgestone Potenza SO2 pole positions all way round as mentioned in previous post. Now the tire experts here in the UK tell me that the pole positions are being discontinued. Furthermore, they are telling me they are not Porsche approved tires as there is no N designation on it. They are recommending SO2 with the N3 designation (no pole position and no Potenza) as a correct tire. They go on saying I also have incorrect tire sizes for my car? I originally had 245/45/16 on the rears and 225/50/16 fronts. I upgraded wheels as recommended at the time to 255/40/17 rears and 225/45/17 fronts. They say I have changed the radius as the 40 and 45 figures refer to 40&45% and dictate the height of the tire in proportion to the width of the tire? What are they talking about? They say I should run 245/40/17 at the rears and 215/45/17 fronts on 17 inch wheels on my car! Guys fire away and give your opinions!
My car is due for new rear tiers. I have Bridgestone Potenza SO2 pole positions all way round as mentioned in previous post. Now the tire experts here in the UK tell me that the pole positions are being discontinued. Furthermore, they are telling me they are not Porsche approved tires as there is no N designation on it. They are recommending SO2 with the N3 designation (no pole position and no Potenza) as a correct tire. They go on saying I also have incorrect tire sizes for my car? I originally had 245/45/16 on the rears and 225/50/16 fronts. I upgraded wheels as recommended at the time to 255/40/17 rears and 225/45/17 fronts. They say I have changed the radius as the 40 and 45 figures refer to 40&45% and dictate the height of the tire in proportion to the width of the tire? What are they talking about? They say I should run 245/40/17 at the rears and 215/45/17 fronts on 17 inch wheels on my car! Guys fire away and give your opinions!
06-09-2003, 03:57 PM
#2
Addict
Rennlist Member
Rennlist Member
Well, your 40 and 45 sidwall size is a bit big for 17's, but it is not a problem. The tire height is actually a bit larger than the stock size (aspect ratio). Your speedo will read a few MPH slower than you are traveling.
225/40/17 front, 255/35/17 rear for mine and all is good.
For tires, hard call. RE-730's are my favorite, but hard to find since they are out of production now.
225/40/17 front, 255/35/17 rear for mine and all is good.
For tires, hard call. RE-730's are my favorite, but hard to find since they are out of production now.
06-09-2003, 06:37 PM
#4
Three Wheelin'
Simon, your tire dealer is correct in saying that your tires are a bit larger in diameter than the stock 951S tires. Stock rear tires are 245/45x16, have a rating of 847 revs/mile (www.tirerack.com, specs for Bridgestone S03 Pole Positions, for example.) This translates to a diameter of 23 7/8". 255/40x17 have a size of 830 revs/mile, or a radius of 24 1/4", which is bigger and slightly affects your gear ratio and speedometer accuracy.
HOWEVER, the stock 951 (non-S) used a stock rear tire size of 225/60x16, which has a rolling radius of 837 revs/mile, or a diameter of 24 1/8", larger than the stock 951S tire size. Thjis is the tire size the gearing and speedo were supposedly calibrated for. In addition, the speedometer was calibrated high. This results in the speedo of a 951S reading several per cent high on stock tires. The tires also look skinny on a 9" wide rim.
Like many people with 17" rims, I run on 255/40x17 rear tires. Mine are Kumhos, which have a larger rolling radius than the Birdgestones, 825 revs/mile. My speedometer error has been reduced to about 1-2% high; in other words, it is better, but still hasn't been fully corrected.
I would recommend that you go with 225/45x17 fronts and 255/40x17 rears; they work just fine. I don't know which tires would be best for you, but I personally wouldn't worry to much about the OE "N" rating on the tires; they are approved for use on new Porsches, but most people seem to run non-N rated tires as replacements with no problems.
HOWEVER, the stock 951 (non-S) used a stock rear tire size of 225/60x16, which has a rolling radius of 837 revs/mile, or a diameter of 24 1/8", larger than the stock 951S tire size. Thjis is the tire size the gearing and speedo were supposedly calibrated for. In addition, the speedometer was calibrated high. This results in the speedo of a 951S reading several per cent high on stock tires. The tires also look skinny on a 9" wide rim.
Like many people with 17" rims, I run on 255/40x17 rear tires. Mine are Kumhos, which have a larger rolling radius than the Birdgestones, 825 revs/mile. My speedometer error has been reduced to about 1-2% high; in other words, it is better, but still hasn't been fully corrected.
I would recommend that you go with 225/45x17 fronts and 255/40x17 rears; they work just fine. I don't know which tires would be best for you, but I personally wouldn't worry to much about the OE "N" rating on the tires; they are approved for use on new Porsches, but most people seem to run non-N rated tires as replacements with no problems.