Correct Tire Size - 986 Forum - The Community for Porsche Boxster & Cayman Owners

RandallNeighbour · 03-19-2006, 03:40 PM

I have aftermarket wheels (ROH) on my boxster. When I bought the car nearly 2 years ago, the Tire Rack guy recommended that I go with 255/40/ZR17 on the back and 225/40/ZR17's on the front. I am now experiencing some cupping on the front tires, and have never been able to get them properly balanced.

My rim sizes are 8.5 x 17 on the rears and 7.5 on the fronts.

I'd love to upgrade to 18's (even though Porsche doesn't recommend it) but I just don't have the cash right now. So I must put more rubber on these rims for another year or so.

Anyone willing to tell me the correct tire size for these rims? I don't want cupping from too wide a tire again, and I think that's what happened, but I'm not sure.

Jeph · 03-19-2006, 06:43 PM

When I had 17s, I had 205s up front. I actually still have those 17" tires (Pirelli PZeros) in my garage. What kind of tires will you be looking at?

TriGem2k · 03-19-2006, 11:16 PM

205/50/17 up front and 255/40/17 rears....OEM 17", doesnt matter about the wheel....However you will be fine with a 225/45/17 up front as well. The tire size should not affect your balancing...This would be due to a bent wheel or defective tire...

insite · 03-20-2006, 03:33 AM

Quote:

Originally Posted by RandallNeighbour

Anyone willing to tell me the correct tire size for these rims? I don't want cupping from too wide a tire again, and I think that's what happened, but I'm not sure.

225 is the correct width for a 7.5" rim. i worked at a tire shop in college; cupping is generally more a function of a poor tread design than tire / wheel width. also, mounting a directional tire to spin the WRONG direction will cause cupping. certain brands / models cupped no matter what; the rest didn't cup at all when properly mounted.

i think the real question is which tires have you been running on your car? btw, i run 225/40 18's up front on 7.5" wheels (as do virtually all base 996's / 997's) and haven't ever had issues w/ cupping.

RandallNeighbour · 03-20-2006, 05:53 AM

I have Kumho MX's on the car now. Cheapest decent tire I could find and afford when I bought the car. I'm thinking of going to Sumitomos next because the other brands are soooo expensive and I seem to need replacements every 18 months or so... and no, I do not spin the tires... but I do like to take U turns at 50 mph when I can get away with it. This sometimes makes the back end kick out a little and I'm sure it wears the side tread a bit each and every time I do it.

But alas, that is the reason I bought the car!

Sumi's come recommended by Jeff (TOOLPANTS) as this is what he chose for his boxster. Insite, did you find that Sumi's cupped?

Thank you for the advice on getting 225's instead of 205's... that's what I suspected.

These are NOT factory rims and therefore, I can't go by what others use on their factory turbo twist rims.

insite · 03-20-2006, 08:13 AM

i don't have personal experience with Kumho MX's; people i talk to really like them. as for the sumitomos, i would avoid them like the plague. i used to use them on my RX-7 (10 yrs ago) and really liked them.

i thought i'd try them on my boxster a year or so back. BAD idea. the sidewalls are so soft that minor steering input at higher speeds will cause the rear end to oscillate back and forth a bit.

aggressive corners are hard to describe: i'd turn in and the tire would roll FAR onto the sidewall. then, mid corner, the tire would recoil off of the sidewall like a rubber band; it wanted to throw the back end TOWARD the apex. these things really scared me. i took them off after a couple of days.

i have tried a lot of different tires on the box. my personal favorite in terms of 'bang for the buck' would be the BF Goodrich KDW2. i LOVE this tire at any price, but it's so CHEAP! i'd also get your alignment checked since that can cause cupping, too. since you've had the tires for 2yrs, though, i doubt this is the issue.

one last thing: guys that race these cars a lot will generally get the same size wheels / tires all the way around (usually 245's on 8.5" wheels). that way, they can rotate the tires and get more life out of the set. this would work for road tires, too, although you may need a slightly larger front sway bar to make it handle the same as w/ staggered wheels & tires.

TriGem2k · 03-20-2006, 09:39 AM

Regardless if they are Factory wheels or not the Boxster recommended tire size for 17" wheels are 205/50/17 front and 255/40/17 rears. However, when i had my 17" i ran 225/45/17 up front as well...

RandallNeighbour · 03-20-2006, 11:07 AM

Insite, and TriGem2k, thanks for your input!

Insite, I wish I had seen your reply 10 minutes earlier. I just bought a set of Sumi's off Tire Rack's web site.

The good thing is that I will probably take the Sumi's off the car by year's end and upgrade to 18's and different rubber. I just wanted the cheapest tire I could put on the car so as to be able to drive it safely between now and then... I don't do a lot of high speed driving, but I will take what you offered into consideration when I do!

nautiquelfe · 03-20-2006, 11:13 AM

what sizes did u put on the front/back of the car? also what was the total shipped to your house?

RandallNeighbour · 03-20-2006, 11:23 AM

Fronts were 225/45/17's <-- which is what I have on the car now... they don't sell 40's in this width.
Backs were 255/40/17's

Sumitomo HRZ II's - $381.25 shipped.

insite · 03-20-2006, 11:30 AM

man, i REALLY think you should try to call them to cancel the order. i cannot stress to you how horrible these tires made my car drive. basically took a 50K porsche and turned it into a geo metro. they were that bad.

now in their defense, they really did make a decent tire back in the day. also, i ran the HTRZ's on my box, not the HTRZ II's. they really did make for a scary drive.

RandallNeighbour · 03-22-2006, 08:42 AM

Insite, just an update...

Tire Rack had already shipped my Sumitomos OUT THE DOOR just 45 minutes after I placed my web order. Dang that's fast! So, I could only refuse the shipment and pay for new tires and new shipping if I wanted to "reverse" the purchase.

The Tire Rack rep told me that the tires do have a 2 week return on them and I should have them mounted, road-force balanced and used for a couple of days and then call back if I am not happy and they'll take them back. That was comforting.

So, I had that done yesterday at a Discount Tire that had a brand new Hunter 9700 machine installed three weeks before.

1) I have ZERO steering wheel or rear end vibration now at any speed (well, under 85 mph as I didn't have time to take it out and run it up over a 100). This is the first time I have driven this car without any wheel shake or vibration. Wow. Those Kumhos were fairly good tires, but evidently they had heavy spots on them that could not be balanced out... causing vibration all the way around.

2) These tires (HRTZ II's) must be build differently from the HRTZ's you found to be dangerous and unpredictable. I took a u turn at about 50 mph and these tires are grippy and didn't give me that rubberband effect you mentioned in your post and on the phone.

BTW, the it might have just been the Tire Rack guy trying to persuade me to stay with my ordered tires, but he didn't have anything positive to say about the tires you recommended to me on the phone... he said they were developed for ricers who wanted a cool looking tread pattern and they were very, very loud...which you mentioned as well.

I'm going to drive the car good and long this weekend and put these cheap tires through the paces and see what they produce or don't produce. We're in for some great cool weather here and I might just get up at 6 am and head out of town to the hill country with the twisty roads to have some fun. I'll report back should I find anything less than I expected.

Should these tires perform well, they are the absolutel cheapest decent tire for a Boxster. I bought a full set and had them road force balanced for $475 out the door.