Changing Tire Aspect Ratio

[Jerry996]

I have a 1999 Carrera 4 and need to replace my rear tires. I'm currently running Continental Contisport Contact 2's, 225-40-18 in front and 285-30-18 in the rear. Since my front tires are at about 50% and the Contisport Contact 2's are discontinued the Tire Rack recommended the Extreme Contact DW tire. The 285-30-18 tire has been on back order for several months now but the 285-35-18 is available. Is this a good choice for me or should I wait longer? I am at the wear bars on the insides of my rear tires and fear that I'm going to get into trouble on a wet day.

Thanks for your time, Jerry

[therock88]

I have a 1999 Carrera 4 and need to replace my rear tires. I'm currently running Continental Contisport Contact 2's, 225-40-18 in front and 285-30-18 in the rear. Since my front tires are at about 50% and the Contisport Contact 2's are discontinued the Tire Rack recommended the Extreme Contact DW tire. The 285-30-18 tire has been on back order for several months now but the 285-35-18 is available. Is this a good choice for me or should I wait longer? I am at the wear bars on the insides of my rear tires and fear that I'm going to get into trouble on a wet day.

Thanks for your time, Jerry

I would not suggest what you indicated above. Changing the ratio you suggested creates about a 1" diameter difference. Try this link here CLICK HERE for your tires. I have used them and many times they beat Tirerack, and ship quickly.

Good luck!

DC

Edited July 12, 2011 by therock88

[Rapewta]

Going from a 285/30-18 to a 285/35-18 will indeed add another inch to the diameter.

You car will sit higher and might even look a little odd. I wouldn't do it.

Good Luck.

[demosan]

I have a 1999 Carrera 4 and need to replace my rear tires. I'm currently running Continental Contisport Contact 2's, 225-40-18 in front and 285-30-18 in the rear. Since my front tires are at about 50% and the Contisport Contact 2's are discontinued the Tire Rack recommended the Extreme Contact DW tire. The 285-30-18 tire has been on back order for several months now but the 285-35-18 is available. Is this a good choice for me or should I wait longer? I am at the wear bars on the insides of my rear tires and fear that I'm going to get into trouble on a wet day.

Thanks for your time, Jerry

Jerry,

Given your car is a C4, having the right size tires is important than a 2WD Carrera. I'm not a tire expert, but if you were to use the 30 profile vs the 35 the difference in revolutions per mile is 836 vs 805 or 3.5%. In order NOT to stress the viscous coupling in your 4WD Carrera, it is advisable to have the MAX difference to be 2%. Therefore, you could be asking for trouble. You mention the front being 50% worn so you might "sneak by the 2% guideline", but if you drive the car in the winter in snow and need the 4WD system to be in good working order I would not take the chance. Demosan

[Dharn55]

Ok I am going to take a little different approach here. The sizes you are running are not stock to begin with. With 18" wheels the OEM sizes are and revs per miles are as follows (all this is based on PS2's but this info is available for most tires on Tirerack.com). 225/40-18 are 829 and 265/35-18 are 822, so the fronts rev 7 more revolutions per miles or .8%. The PS2 is not made in a 285/30-18 but the 295/30-18 is 832, so the front revolve -3 or .4%. obviously the 285/30-18 would be smaller but I am not sure how much. The 285/35-18 is not made in the PS2.

I have run 235/40-18 fronts at 819 and 265/40-18 at 790 for several years with no problems. These are a difference of 29 revs per mile or 3.5%.

When you are evaluating these tires you should really look at the size/revs per mile and stay as close to the original as possible, although I have been told that 5% or less is OK. I know that starting in year 2002 (I think) the rears were changed to 285/30-18. I don't know if the PSM or the ABS computers were reprogrammed at that time.

By the way the Conti Sport Contact2 seems to be available in 295/30-18 at 827 revs with the 225/40-18 at 825 revs, so only 2 revs per mile different. But they are also much more expensive than the DW's.

Hope this may be helpful.

[xmac]

Ok I am going to take a little different approach here. The sizes you are running are not stock to begin with. With 18" wheels the OEM sizes are and revs per miles are as follows (all this is based on PS2's but this info is available for most tires on Tirerack.com). 225/40-18 are 829 and 265/35-18 are 822, so the fronts rev 7 more revolutions per miles or .8%. The PS2 is not made in a 285/30-18 but the 295/30-18 is 832, so the front revolve -3 or .4%. obviously the 285/30-18 would be smaller but I am not sure how much. The 285/35-18 is not made in the PS2.

I have run 235/40-18 fronts at 819 and 265/40-18 at 790 for several years with no problems. These are a difference of 29 revs per mile or 3.5%.

When you are evaluating these tires you should really look at the size/revs per mile and stay as close to the original as possible, although I have been told that 5% or less is OK. I know that starting in year 2002 (I think) the rears were changed to 285/30-18. I don't know if the PSM or the ABS computers were reprogrammed at that time.

By the way the Conti Sport Contact2 seems to be available in 295/30-18 at 827 revs with the 225/40-18 at 825 revs, so only 2 revs per mile different. But they are also much more expensive than the DW's.

Hope this may be helpful.

Dharn55,

Thanks for the help and noting the 295's are actually a closer in revs. I too have a 99 C4 and am in the exact same situation. Unfortunately in my case, I hit something on the freeway which damaged one of my rear tires and the fronts are in need of replacement. I too was looking at the Extreme DW's but they are on backorder. I am considering the Sumitomo HTRZ III's since these would be for daily driving.

[xmac]

I just got off the phone with a rep from Discount Tire. He said the front (225's) are at least 45 days from coming in and the rear (285's) are indefinite. He said the tires are made in Portugal and they are having difficulties in producing enough tires or maybe they are not producing those sizes right now??

In any case, it looks like I need to pick another tire or park the car.

[Dharn55]

I have been thinking seriously about the new Michelin PSS (Pilot Super Sports) which are the "replacement" for the PS2. They are more than the Conti DW's but less than the PS2's. If you read the reviews they say they are better than the PS2's, which have been rated one of the best for the 996 if price is not an object (I know I have loved mine) and are supposed to last longer (higher wear rating). I know that Costco offered to order them for me and they were about $1,075 installed after the $70 rebate. Something to think about.

[xmac]

As much as I would have preferred the Conti's, I ended up going with the Hankook V12 EVO's in 295/30/18 & 225/40/18, should be here next week.