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Old 05-21-2019, 03:29 AM   #1
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Correct tire size question

I recently purchased a 1999 boxster base. It came with wheels I believe came off a cayman. The tire sizes are front 235/40 ZR18. Rear 265/40ZR18. I checked the speed with my gps and itís within 1 mph of what the speedometer reads. Are these tires too wide? No problems with any kind of rubbing on the frame or fenders.
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Old 05-21-2019, 03:39 AM   #2
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I've Cayman wheels on my 2003. I fitted spacers to front to give them out more for aesthetics than anything else.

The only thing I would say is that the car is over tyred now so handling is not as precise but for fast road use it's no problem. Track use I would fit back on the original 17's

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Old 05-21-2019, 03:59 AM   #3
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Thanks for the information. I like the way the tires and wheels fill out the wheel wells, I just wasnít sure if these sizes would cause problems. I see there are 18Ē tires listed for 1999 boxsters on tire rack but the sizes are different 1999****PORSCHE****BOXSTER
Front: 225/40ZR18 92Y XL
Rear: 265/35ZR18 97Y XL
Can these go on these rims?
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Old 05-21-2019, 04:04 AM   #4
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Originally Posted by wrs986 View Post
I recently purchased a 1999 boxster base. It came with wheels I believe came off a cayman. The tire sizes are front 235/40 ZR18. Rear 265/40ZR18. I checked the speed with my gps and itís within 1 mph of what the speedometer reads. Are these tires too wide? No problems with any kind of rubbing on the frame or fenders.
Speedo is fine, no rubbing and you're asking if the tires are too wide? Apparently they are not too wide. Only Issue I see with your combination is the rear should have been 265/35/18 to give the tires a almost equal rotational speed.
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Old 05-21-2019, 05:57 AM   #5
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Originally Posted by wrs986 View Post
Thanks for the information. I like the way the tires and wheels fill out the wheel wells, I just wasnít sure if these sizes would cause problems. I see there are 18Ē tires listed for 1999 boxsters on tire rack but the sizes are different 1999****PORSCHE****BOXSTER
Front: 225/40ZR18 92Y XL
Rear: 265/35ZR18 97Y XL
Can these go on these rims?
Yes that is the proper size & you will be amazed at the increased acceleration from not having a tire that is too tall. You don't need the XL either (Xtra load)
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Old 05-21-2019, 06:02 AM   #6
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Your speedometer is probably more accurate than most Porsches. Generally German cars are 2-3 mph higher than actual speed. Even though the speedometers read high, the odometers are dead nuts on. Yours may be a little affected with the rear tires, but not much.
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Old 05-21-2019, 07:52 AM   #7
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I would swap out for the proper sizes as soon as possible and enjoy the summer. if you wont drive in the winter at all, get some soft sticky ones.

Front: 225/40ZR18
Rear: 265/35ZR18

While I prefer the 18' turbo twists, these are really nice looking wheels, and look easy to clean as well. I this is your first 986, welcome. Your going to have fun tooling around. :+)
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Old 05-21-2019, 12:07 PM   #8
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Originally Posted by tonythetiger View Post
I would swap out for the proper sizes as soon as possible and enjoy the summer. if you wont drive in the winter at all, get some soft sticky ones.

Front: 225/40ZR18
Rear: 265/35ZR18

While I prefer the 18' turbo twists, these are really nice looking wheels, and look easy to clean as well. I this is your first 986, welcome. Your going to have fun tooling around. :+)
Surely you're pulling his leg and just forgot the wink emoji, right?

Variance on front tire 40 series in 225 vs 235 is 4mm... ummm 1/8 of an inch. That's rounding or ummm tire wear - HA!

Difference b/w 40 and 35 series on rear 265 tire is theoretically 13.25 mm...barely 1/2in (assuming manufactured w/ 0% tolerance variance - LOL). And that's if, and ONLY IF, 35 series is even available (it may well not be and mfg could hedge this a bit to keep sku counts down). Not to mention a '99 Boxster does not have nearly the current day electronic "nannies" which could (though very doubtfully) be even affected ~ 1/2" variance (again, sans tire wear).

Again, hope you know him and meant that post as tongue in cheek. If not I'd suggest seeking a better understanding of tire sizes.
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Old 05-21-2019, 01:29 PM   #9
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Originally Posted by Burg Boxster View Post
Surely you're pulling his leg and just forgot the wink emoji, right?

Variance on front tire 40 series in 225 vs 235 is 4mm... ummm 1/8 of an inch. That's rounding or ummm tire wear - HA!

Difference b/w 40 and 35 series on rear 265 tire is theoretically 13.25 mm...barely 1/2in (assuming manufactured w/ 0% tolerance variance - LOL). And that's if, and ONLY IF, 35 series is even available (it may well not be and mfg could hedge this a bit to keep sku counts down). Not to mention a '99 Boxster does not have nearly the current day electronic "nannies" which could (though very doubtfully) be even affected ~ 1/2" variance (again, sans tire wear).

Again, hope you know him and meant that post as tongue in cheek. If not I'd suggest seeking a better understanding of tire sizes.
I agree his statement was overboard, however if it was my car I would go with 265/35/18 tires to better match the rotation speed of the front tires and make the car feel less twitchy and yes most all tire manufacturers make that tire size. Also the circumference difference between the the 35 and 40 series is a 3.9% smaller circumference for the 35 series. So he would get a bit quicker acceleration and stiffer sidewall for better handling, but will ride just a bit rougher.
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Old 05-21-2019, 04:23 PM   #10
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I agree his statement was overboard, however if it was my car I would go with 265/35/18 tires to better match the rotation speed of the front tires and make the car feel less twitchy and yes most all tire manufacturers make that tire size. Also the circumference difference between the the 35 and 40 series is a 3.9% smaller circumference for the 35 series. So he would get a bit quicker acceleration and stiffer sidewall for better handling, but will ride just a bit rougher.
To which front tire size - current actual or should be?

I highly doubt wrs986's IMSA or F1 career is in jeopardy b/c of size variances in the tires he has vs what should be there. Nor his Spec B career (well mostly b/c he'd be in violation running 18" ). Not to mention he's wearing matching shoes w/ tread on them which candidly is better than probably half the vehicles on road... and 99.9% better than every Kia or Hyundai I see traveling 95MPH+ in the rain - LOL

Would I consider correct sizes when replacing tires...probably, but it's not imperative b/c he really is close enough. You're talking about an overall ~ 2.9% variance delta b/w recommended size circumference vs what's actually on there (it's 3.8% F/R w/ what on their vs .9% F/R for recommended). Yes, I know that size should be available. Who knows though, maybe Tire Kingdom or whomever was out of stock on them the week the previous owner needed new rubber.

So yes, your mathematics are correct on the rotational speed of the tires. Precision is one thing... accuracy another. Think wrs, me or you would notice? Heck, I'll buy you steak dinner (or tofu if that's your protein) w/ all the trimmings if you could prove to me w/ even 50% accuracy using only your "buttometer" which tire size was on any vehicle w/ variances to this level.

Bottom line, wrs986 came here to make sure what he has works. It does... and well too! Let's encourage him to not fret over this non-issue but instead to get out there and enjoy his new ride w/ those great looking wheels and see what these cars are all about and truly capable of. That's what this should be all about.

Good luck
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Old 05-21-2019, 05:20 PM   #11
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Originally Posted by Burg Boxster View Post
To which front tire size - current actual or should be? )
The current actual front tires on the car.



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Originally Posted by Burg Boxster View Post
So yes, your mathematics are correct on the rotational speed of the tires. Precision is one thing... accuracy another. Think wrs, me or you would notice? Heck, I'll buy you steak dinner (or tofu if that's your protein) w/ all the trimmings if you could prove to me w/ even 50% accuracy using only your "buttometer" which tire size was on any vehicle w/ variances to this level.
You owe me a steak dinner then. When I have to go from Summer to Winter tires I can tell a big difference in the twitchyness of the car. The front tires are smaller in circumference and turn in very quickly and the rear tires turning at a slower rate are trying to catch up. I absolutely hate the front tires spinning faster then the rear tires, especially in a mid engine car and I can tell the difference, very unbalanced.

This is what he is having to deal with when he could have the above mentioned nearly identical rotation speed and have a much more pleasant handling experience.



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Bottom line, wrs986 came here to make sure what he has works. It does... and well too! Let's encourage him to not fret over this non-issue but instead to get out there and enjoy his new ride w/ those great looking wheels and see what these cars are all about and truly capable of. That's what this should be all about.
Yes his setup works, not optimal, not even as close to factory level of rotation speed difference (if that's the correct tire size Tire Rack and tonythetiger said to go with), but works well enough for him I guess. I'm hope his tire combination will give him plenty of joy, but he is asking questions about his tires so we could be assuming his happiness level wrongly.

I just said if it was my car, what I would do with it, which would be cheaper then what tonythetiger suggested for him to do and balance out the car better.

.........................My Summer tire setup........................................My Winter tire setup.



It's like I'm driving a different car. Yes someone swiped the original Carrera Light 18" wheels off the car at some point, probably one of the dealerships it was at.
I'm the 4th owner of the car.
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Old 05-21-2019, 06:02 PM   #12
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I appreciate all the information on the tire sizes. Iím the 3rd owner of this car and the previous owner did not put these wheels on the car. When I went to tire rackís website it has listed for a 2006 PORSCHE CAYMAN S WITH STANDARD BRAKES WITH TPMS
Front: 235/40R18 95W XL
Rear: 265/40R18 101W XL
Same sizes as mine. I know the cayman is the 987 generation but I thought the chassis, wheels etc were similar to the 986.
The tires are 12 years old, I plan to get new ones soon and I wanted to make sure if I decided to get the proper size tires for this car that they would fit on these rims.
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Old 05-21-2019, 06:20 PM   #13
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Originally Posted by wrs986 View Post
I appreciate all the information on the tire sizes. I’m the 3rd owner of this car and the previous owner did not put these wheels on the car. When I went to tire rack’s website it has listed for a 2006 PORSCHE CAYMAN S WITH STANDARD BRAKES WITH TPMS
Front: 235/40R18 95W XL
Rear: 265/40R18 101W XL
Same sizes as mine. I know the cayman is the 987 generation but I thought the chassis, wheels etc were similar to the 986.
The tires are 12 years old, I plan to get new ones soon and I wanted to make sure if I decided to get the proper size tires for this car that they would fit on these rims.
Wow! 12 years old! Yikes!

Get 235/40/18 front and 265/35/18 rear.

Want a decent sent of Summer tires without breaking the bank, get these. I run them on my car and love them.


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Old 05-21-2019, 07:00 PM   #14
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12 year old tires!?!?! Dang, that's gotta be close to some sort of record around here. On a bright side, those wheels must be in good shape!

Were this my car... before I installing new tires I'd have fun finding an autocross or car control clinic where I could go tear those puppies up for a day/afternoon (if I could get the vehicle there safely w/o risk... i.e. towing as there's plenty of risk driving on 12yr old tires). Those events are most often held in extremely low probability of incident zones (huge pads of asphalt/concrete w/o obstructions like buildings, light posts, curbs, etc). Then you can really learn what sloppy handling tires feel like on a precise Porsche instrument. You'll come to appreciate the new tires in correct size for you new baby immediately following.

Congrats and drive her in good health!
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Old 05-21-2019, 07:41 PM   #15
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Originally Posted by Burg Boxster View Post
Surely you're pulling his leg and just forgot the wink emoji, right?



Variance on front tire 40 series in 225 vs 235 is 4mm... ummm 1/8 of an inch. That's rounding or ummm tire wear - HA!



Difference b/w 40 and 35 series on rear 265 tire is theoretically 13.25 mm...barely 1/2in (assuming manufactured w/ 0% tolerance variance - LOL). And that's if, and ONLY IF, 35 series is even available (it may well not be and mfg could hedge this a bit to keep sku counts down). Not to mention a '99 Boxster does not have nearly the current day electronic "nannies" which could (though very doubtfully) be even affected ~ 1/2" variance (again, sans tire wear).



Again, hope you know him and meant that post as tongue in cheek. If not I'd suggest seeking a better understanding of tire sizes.


Hey now, strongly worded in review of my own words, yes; I said ASAP and didnít mean it.. These tires are fine and enjoy but every time I fall into a car I think about what I want to change. Tires are obvious. I like soft sticky rubber and was sharing my thoughts. Iíll stay with stock sizes cuz someone smarter then me figured that out.
Enjoy the ride, keep talking here and distill what we say to where it makes sense for you. Steer to slide.


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Old 05-21-2019, 08:09 PM   #16
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Originally Posted by wrs986 View Post
I appreciate all the information on the tire sizes. Iím the 3rd owner of this car and the previous owner did not put these wheels on the car. When I went to tire rackís website it has listed for a 2006 PORSCHE CAYMAN S WITH STANDARD BRAKES WITH TPMS
Front: 235/40R18 95W XL
Rear: 265/40R18 101W XL
Same sizes as mine. I know the cayman is the 987 generation but I thought the chassis, wheels etc were similar to the 986.
I recently purchased a set of Cayman wheels/tires for my Ď02 base with the same tire set up as the OP.
No issues other than now the speedometer is 3mph slower than actual speed whereas it used to be 5mph slower, and I had to adjust the headlights to compensate for the taller tires in the rear.
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Old 05-21-2019, 09:03 PM   #17
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Wow, 12yr old tires... you will be amazed how much better your car feels after you get new tires. Personally, I wouldn’t do any “spirited” driving on tires that old. Get them replaced as soon as you can, then get after it. Just go with a tire profile where both are as close to the proper stock overall diameter and you’ll be fine. Too wide? Never. (Sometimes, maybe) I run 19” 9.5” fronts and 10.5” rears. Love it.
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Old 05-21-2019, 10:01 PM   #18
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Yes, mount some fresh tires ASAP because 12 yr old tires suck! Ideal sizes for those wheels on a 986 are 235/40/18 and 265/35/18.

Pretty fun discussion on tire diameter differences though. "Twitchiness"??

All 987, 997, 981, 991 run 1" smaller diameter in front than the rear. It adds a little rake to the car so there is more weight on the front tires. This helps to generate excellent car balance in the corners. The idea that all these cars are "twitchy" because of the different diameter has never been discovered until now.

If your car feels twitchy, it needs an alignment.
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Old 05-22-2019, 02:11 AM   #19
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This car was so well cared for but the previous owner was not a ďcar guyĒ and I figured the tires would need to be replaced ASAP.
How far off will my speedometer be if I go from 265/40/18 to 265/35/18?
Right now itís within 1 mph of my GPS. Is the speedometer reading taken from the rear or front tires? Iíll run down to tire rack next week and replace the Potenzas. What was oem for this car?
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Old 05-22-2019, 03:22 AM   #20
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...Were this my car... before I installing new tires I'd have fun finding an autocross or car control clinic where I could go tear those puppies up for a day/afternoon (if I could get the vehicle there safely w/o risk... i.e. towing as there's plenty of risk driving on 12yr old tires). Those events are most often held in extremely low probability of incident zones (huge pads of asphalt/concrete w/o obstructions like buildings, light posts, curbs, etc). ...
Good advice to tow the car, but I would never suggest to anyone to take a car with old tires to an autocross and 'tear those puppies up'. When old (and most likely dry-rotted) tires blow, they're likely to shred and tear up the fender or the rear quarter wheel-well, depending on which tire blows.

On a side note, check out this video. You may have to change your signature.

https://www.youtube.com/watch?v=POSKNO7z5Sw
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