Tire Question

[BocaTarga]

I am in need of replacing my rear tires but upon reviewing the OEM specs I noticed that the front and rear have different tire wall sizes (i.e. Bridgestone Pontenza S-02A (Rear) 285/30 ZR18 (Front) 225/40 ZR18). The issue here is the 30 vs 40. Does anyone know why the front tires are bigger (40) than the rear (30)? Must I replace the rear with 30's or can I replace them with the same size (40) as the front?

Anyone know what Porsche was thinking about these two different sizes?

[demosan]

I am in need of replacing my rear tires but upon reviewing the OEM specs I noticed that the front and rear have different tire wall sizes (i.e. Bridgestone Pontenza S-02A (Rear) 285/30 ZR18 (Front) 225/40 ZR18). The issue here is the 30 vs 40. Does anyone know why the front tires are bigger (40) than the rear (30)? Must I replace the rear with 30's or can I replace them with the same size (40) as the front?

Anyone know what Porsche was thinking about these two different sizes?

contact

BT,

Short answer is the larger rear tires ( lower profile) are to provide a larger patch ( traction) in the rear to compensate for the heavier weight distribution being in the rear of a rear engine porsche. The rolling diameter of the front and rear tires are the same.

Demosan

[Porschelibrarian]

Also if you check the wheel sizes are different too. It has nothing to do with the Targa style. You would have to get a new set of wheels to match the ones on the front so the wheels and tires match. The advantage would be to do a front to back tire rotation and the tires should last a bit longer. However you will loose alot of the handling and ride qualities. It makes more sense to do a little research into the tires and buy a set with a higher treadlife rating. For example the Bridgestone Potenza RE960AS Pole Position have a treadlife rating of 400 and are rated as AAA and should be about 800 for a set and the Bridgestone Potenza RE050A Pole Position have a treadlife rating of 140 and are also rated as AAA and a set of 4 should be about 950. The former should last about 2.5 times longer

BTW check out Value Tire in Hallandale, Florida. I use them and have sent a couple of other Porsche owners there. They usually have good prices. Call ahead to get a price.

Edited January 5, 2008 by Porschelibrarian

[BocaTarga]

I am in need of replacing my rear tires but upon reviewing the OEM specs I noticed that the front and rear have different tire wall sizes (i.e. Bridgestone Pontenza S-02A (Rear) 285/30 ZR18 (Front) 225/40 ZR18). The issue here is the 30 vs 40. Does anyone know why the front tires are bigger (40) than the rear (30)? Must I replace the rear with 30's or can I replace them with the same size (40) as the front?

Anyone know what Porsche was thinking about these two different sizes?

contact

BT,

Short answer is the larger rear tires ( lower profile) are to provide a larger patch ( traction) in the rear to compensate for the heavier weight distribution being in the rear of a rear engine porsche. The rolling diameter of the front and rear tires are the same.

Demosan

Sorry, but the "rolling diameter" is NOT the same. The diameter of the rear's are about 1/2 inch shorter. Check your own car using a level and measure to the pavement from the top of the tire. With the same diameter rims, which is OEM, it stands to be logical that the front's with a "/40" vs "/30" wall height would higher. I am not interesed in changing the rims since I do not intend to change tire width, but am wondering why the front's would be designed to us a higher wall, hence larger diameter?

[BocaTarga]

Also if you check the wheel sizes are different too. It has nothing to do with the Targa style. You would have to get a new set of wheels to match the ones on the front so the wheels and tires match. The advantage would be to do a front to back tire rotation and the tires should last a bit longer. However you will loose alot of the handling and ride qualities. It makes more sense to do a little research into the tires and buy a set with a higher treadlife rating. For example the Bridgestone Potenza RE960AS Pole Position have a treadlife rating of 400 and are rated as AAA and should be about 800 for a set and the Bridgestone Potenza RE050A Pole Position have a treadlife rating of 140 and are also rated as AAA and a set of 4 should be about 950. The former should last about 2.5 times longer

BTW check out Value Tire in Hallandale, Florida. I use them and have sent a couple of other Porsche owners there. They usually have good prices. Call ahead to get a price.

Please see my response earlier. The wheel diameters are NOT the same. This is why I am puzzled.

If I am not mistaken, the "285/" and "225/" refer to the width. I am not interested in putting the same tire widths on front and back. I am interested in understanding why the difference in the tire "wall" heights. In other words I would like to put "285/40" on back instead of the recommended "285/30". This is not what is spec'ed and I wish someone could explain why?

Thanks

Edited January 6, 2008 by BocaTarga

[Q-Ship986]

aaa / bb R cc

aaa = tire width (not tread width), exclusive of ornamentation (i.e. "whitewall" stripe or raised lettering) when tire is mounted on a theoretical rim of a certain size (depends on the tire).

bb = aspect ratio of section height to tire width, in other words, (aaa)(bb)=section height. This height includes the bead portion, which when the tire is mounted, is hidden.

cc = nominal rim diameter, in inches

---------------------

So, 285/30R18 would have a strictly calculated section height of 85.5mm.

225/40R18 has a strictly calculated section height of 90.0mm.

For a difference of 4.5mm (about 0.18 inch).

Tire industry standards have a little rounding in them, and tire manufacturers may spec slightly different values, depending on design, but the above are the basics.

In this case, the 40-series tire is "taller," but if the 30-series tire was a bit wider, say a 305/30R18 (if there is such a thing), then the 30-series tire would be "taller." You can't judge strictly by the aspect ratio.

As for PAG selecting these sizes, the choices are typically made based on load capacity and desired vehicle performance attributes.

--Brian

Edited January 7, 2008 by Q-Ship986

[demosan]

I am in need of replacing my rear tires but upon reviewing the OEM specs I noticed that the front and rear have different tire wall sizes (i.e. Bridgestone Pontenza S-02A (Rear) 285/30 ZR18 (Front) 225/40 ZR18). The issue here is the 30 vs 40. Does anyone know why the front tires are bigger (40) than the rear (30)? Must I replace the rear with 30's or can I replace them with the same size (40) as the front?

Anyone know what Porsche was thinking about these two different sizes?

contact

BT,

Short answer is the larger rear tires ( lower profile) are to provide a larger patch ( traction) in the rear to compensate for the heavier weight distribution being in the rear of a rear engine porsche. The rolling diameter of the front and rear tires are the same.

Demosan

Sorry, but the "rolling diameter" is NOT the same. The diameter of the rear's are about 1/2 inch shorter. Check your own car using a level and measure to the pavement from the top of the tire. With the same diameter rims, which is OEM, it stands to be logical that the front's with a "/40" vs "/30" wall height would higher. I am not interesed in changing the rims since I do not intend to change tire width, but am wondering why the front's would be designed to us a higher wall, hence larger diameter?

BT,

You are correct that there may be a slight difference in "rolling diameter" in some of the standard factory fitments of tires, although the difference is not as significant as you think as the sidewall height is theortically calculated by the product of width and aspect ratio. For example, for my C4S, the standard tires are 295 x30 rears and 225 x40 fronts (18's both axles). The sidewall height of the fronts would be calculated as 225x.40 or 90mm. Rears sidewall height would be 295 x 30 or 88.5mm. SO you are correct there is a difference of 1.5mm, which is within the guidelines of being with 1-2% of theortical to ensure the AWD performance of the drivetrain is not compromised. I am not trying to sell myself as a tire fitment expert here as I had some of the same questions when I replaced the tires recently on my C4S. If you go to the tirerack.com website, there are several articles dealing with Pcar fitement issues that you may find helpful.

Regards, demosan :cheers:

[BocaTarga]

I am in need of replacing my rear tires but upon reviewing the OEM specs I noticed that the front and rear have different tire wall sizes (i.e. Bridgestone Pontenza S-02A (Rear) 285/30 ZR18 (Front) 225/40 ZR18). The issue here is the 30 vs 40. Does anyone know why the front tires are bigger (40) than the rear (30)? Must I replace the rear with 30's or can I replace them with the same size (40) as the front?

Anyone know what Porsche was thinking about these two different sizes?

contact

BT,

Short answer is the larger rear tires ( lower profile) are to provide a larger patch ( traction) in the rear to compensate for the heavier weight distribution being in the rear of a rear engine porsche. The rolling diameter of the front and rear tires are the same.

Demosan

Sorry, but the "rolling diameter" is NOT the same. The diameter of the rear's are about 1/2 inch shorter. Check your own car using a level and measure to the pavement from the top of the tire. With the same diameter rims, which is OEM, it stands to be logical that the front's with a "/40" vs "/30" wall height would higher. I am not interesed in changing the rims since I do not intend to change tire width, but am wondering why the front's would be designed to us a higher wall, hence larger diameter?

BT,

You are correct that there may be a slight difference in "rolling diameter" in some of the standard factory fitments of tires, although the difference is not as significant as you think as the sidewall height is theortically calculated by the product of width and aspect ratio. For example, for my C4S, the standard tires are 295 x30 rears and 225 x40 fronts (18's both axles). The sidewall height of the fronts would be calculated as 225x.40 or 90mm. Rears sidewall height would be 295 x 30 or 88.5mm. SO you are correct there is a difference of 1.5mm, which is within the guidelines of being with 1-2% of theortical to ensure the AWD performance of the drivetrain is not compromised. I am not trying to sell myself as a tire fitment expert here as I had some of the same questions when I replaced the tires recently on my C4S. If you go to the tirerack.com website, there are several articles dealing with Pcar fitement issues that you may find helpful.

Regards, demosan :cheers:

Ok, thanks. You guys are great input. I appreciate the help.

Going with OEM specs.

[KevinMac]

I am in need of replacing my rear tires but upon reviewing the OEM specs I noticed that the front and rear have different tire wall sizes (i.e. Bridgestone Pontenza S-02A (Rear) 285/30 ZR18 (Front) 225/40 ZR18). The issue here is the 30 vs 40. Does anyone know why the front tires are bigger (40) than the rear (30)? Must I replace the rear with 30's or can I replace them with the same size (40) as the front?

Anyone know what Porsche was thinking about these two different sizes?

contact

BT,

Short answer is the larger rear tires ( lower profile) are to provide a larger patch ( traction) in the rear to compensate for the heavier weight distribution being in the rear of a rear engine porsche. The rolling diameter of the front and rear tires are the same.

Demosan

Sorry, but the "rolling diameter" is NOT the same. The diameter of the rear's are about 1/2 inch shorter. Check your own car using a level and measure to the pavement from the top of the tire. With the same diameter rims, which is OEM, it stands to be logical that the front's with a "/40" vs "/30" wall height would higher. I am not interesed in changing the rims since I do not intend to change tire width, but am wondering why the front's would be designed to us a higher wall, hence larger diameter?

BT,

You are correct that there may be a slight difference in "rolling diameter" in some of the standard factory fitments of tires, although the difference is not as significant as you think as the sidewall height is theortically calculated by the product of width and aspect ratio. For example, for my C4S, the standard tires are 295 x30 rears and 225 x40 fronts (18's both axles). The sidewall height of the fronts would be calculated as 225x.40 or 90mm. Rears sidewall height would be 295 x 30 or 88.5mm. SO you are correct there is a difference of 1.5mm, which is within the guidelines of being with 1-2% of theortical to ensure the AWD performance of the drivetrain is not compromised. I am not trying to sell myself as a tire fitment expert here as I had some of the same questions when I replaced the tires recently on my C4S. If you go to the tirerack.com website, there are several articles dealing with Pcar fitement issues that you may find helpful.

Regards, demosan :cheers:

Ok, thanks. You guys are great input. I appreciate the help.

Going with OEM specs.

Here is a great calculator to see tire specs;

http://www.miata.net/garage/tirecalc.html