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 it’s 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.

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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Thanks for the information. I like the way the tires and wheels fill out the wheel wells, I just wasn’t sure if these sizes would cause problems. I see there are 18” 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?

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.

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)

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.

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.

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? :rolleyes:

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 :)

The current actual front tires on the car.

https://i.postimg.cc/xC2s54D1/tires.jpg

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.

https://i.postimg.cc/jdxSmxQz/tires2.jpg

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.

https://i.postimg.cc/QtkZ8Cwg/summertires.jpg https://i.postimg.cc/MKGJwWDR/wintertires.jpg

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. :mad:
I'm the 4th owner of the car.

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.

https://i.postimg.cc/4yf6M2pv/tr5.jpg
https://i.postimg.cc/8z66ygqQ/tr4.jpg

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!

Hey now, strongly worded in review of my own words, yes; I said ASAP and didn’t 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. I’ll 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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I recently purchased a set of Cayman wheels/tires for my ‘02 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.

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.

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"?? :rolleyes:

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. :cheers:

If your car feels twitchy, it needs an alignment.

This car was so well cared for but the previous owner was not a “car guy” 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 it’s within 1 mph of my GPS. Is the speedometer reading taken from the rear or front tires? I’ll run down to tire rack next week and replace the Potenzas. What was oem for this car?

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. :D:D:D

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

Neat vid - thanks. But change my signature? Not. A. Chance.

That ain't no Pius... it's a hellcat in sanctimonious HOV lane cladding. :D

Well you learned something today again then. On a alignment for street car, the tire rotation makes a big difference when you decide to go for a spirited drive. No my car does not need an alignment as I can fell the twitchyness changing the front tire to a smaller faster rotating speed.

People that race, align their cars much different then street cars. Alignments good for race cars are not good for street cars, if you want to have proper tire wear.

"All 987, 997, 981, 991 run 1" smaller diameter in front than the rear."? You are completely wrong in this statement.

The speedometer reading comes from averaging the abs speed sensors from all 4 wheels I believe. I read it on the internet so it must be true! If you use the stock 18 inch tire sizes your speedometer will read about 2-3 mph higher than what a mobile phone GPS speedometer app is showing. There is an EU regulation or German law that fines auto manufacturers for all cars they sell whose speedometers read lower than actual speed. We've owned 1 Audi, 3 BMW's and 1 Porsche and that's always been our speedometer experience with German cars. And then there is a US law that requires all auto manufacturers to have accurate odometers so the warranties don't run out sooner than they should. So with the right sized tires your speedometer is going to read a little higher than actual speed and your odometer is going to be accurate.

Negative ghostrider, stock 981 Boxsters typically run 26" diameter in front and 27" diameter in rear. I ride right seat in a LOT of 987/997/981/991 cars at the race track as an instructor. No twitch, no unusual tire wear as long as the alignment was set up for the type of driving they do.

I drove my street 986 for 10 years and 100 track days experimenting with a lot of different tire sizes, no twitch unless I had too much rear sway bar. :cool:

Here are stock sizes for a 2013 Boxster. Note the difference F/R in diameter and revs per mi. Different sizes, no twitch.
https://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+R E-71R&frontTire=345WR8RE71R&rearTire=645WR8RE71RXL&v ehicleSearch=true&fromCompare1=yes&autoMake=Porsch e&autoYear=2013&autoModel=Boxster&autoModClar=


My 2009 Cayman runs 25"F, 26"R as stock sizes.
https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Sport+ 4S&frontTire=34YR8PS4SXL&rearTire=64YR8PS4SXL&vehi cleSearch=true&fromCompare1=yes&autoMake=Porsche&a utoYear=2009&autoModel=Cayman&autoModClar=

Here are stock sizes for a 987.2 Carrera. 25"F, 26" R
https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Sport+ PS2&frontTire=34YR8SPTPS2XLN4&rearTire=64YR8SPORTP S2XLN4&vehicleSearch=true&fromCompare1=yes&autoMak e=Porsche&autoYear=2009&autoModel=911%20Carrera%20 Coupe&autoModClar=

I think somebody has been yanking your chain a bit KRAM.

Not negative. Just because a number goes from 26 to 27 does not make it a full 1", nor does your statement that ALL 987, 997, 981, 991 run 1" smaller diameter in front than the rear hold true.

I can tell the difference, you can't, no big deal.

Only person yanking chains is you. My Boxster is my DD and it's my only vehicle, it's been that way since 2014. I notice everything on my car when something changes. I personally do not like faster rotating front tires, it throws off the balance to me.

I'll will always recommend getting your tires to rotate as close as possible to the same speed. Nothing wrong or illogical in my recommendation and you're not going to change that.

Carry on KRAM... :rolleyes:

You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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Oh ya this is spark plug horsepower guy. nuff said. :)

Change it on my car and I will notice the change without being told.

Why are you even commenting? Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car? Or are you just being a chain jerkier like Topless?

Because people use these forums as a knowledge base. If ridiculousness is left unchallenged, it gets defacto credibility.

That's why I'm commenting.

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Driving blindfolded. Now there's an idea! Show us how it's done, Maytag! :D:D:D:D

You said nothing to discredit my recommendation. You've put nothing in check. You're just showing you want to argue just like Topless with zero bases to argue from.

https://beta.washingtonpost.com/technology/2019/01/12/driving-blindfolded-bird-box-challenge-just-dont-officials-say/?outputType=amp


Can't make this stuff up, Haha

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C'mon KRAM, put up or shut up.

Lets see hard data for all the 987/997/981/991 cars that came from the Porsche factory with identical or nearly identical (less than .3" difference) F/R tire diameters. Google is your friend and there are a few, and I know which ones. The numbers are out there for all to see. If you are going to make an argument AT LEAST be able to support that argument with hard facts instead of butt dyno mumbo jumbo.

You have 30 minutes to demonstrate your awesomeness to all who visit here.

Go!

I don't have 30 min to do anything.

You made the false claim. Two of you examples were not 1" different.

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?

Yes, glad you asked. There is a very valid reason. On 987,981, and most 997 cars the PSM and ABS are precisely calibrated to stock tire stagger in diameter (typically 1" differential). Put the same diameter tire on these cars and the PSM overreacts in corners applying the rear brakes and cutting the throttle. It can also confuse ABS creating an "ice pedal" condition under heavy braking causing a crash.

Getting the factory tire diameter stagger is very important in Porsche cars after 2005 so that PSM and ABS function properly. Mounting the same diameter tire on all cars without consideration of factory designed stagger is foolish and dangerous. The 986 has a much simpler PSM and ABS that do not require precise matching of tire diameter. The OP can happily run the tires he is on with little or no effect. If he does change the rear to a 265/35/18 he will notice a bit better acceleration and his speedo will again be off by 4-6mph.

OMG! LOL I hope this doesn't spread like the Tide Pod challenge! Good lord, were we that stupid then?

Hey man, don't start throwing out squirrels here..... you're changing the direction to deflect....
This is about whether or not you can FEEL it..... hahaha.

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I said street cars, not race cars were "ice pedal" is a common issue and I have yet to read any racers blaming it on the tire circumference size. I've read racers having a too aggressive master cylinder setups along with running tires that are basically slicks, to pads and rotors are not stopping the rotation of your wheels enough to reach lockup.

PCA even made a 2019 the proposed rule change. "Every ABS whose PWIS programming includes an option for a PCCB flash may use that flash. Doing this seems to help with "ice" pedal and is inexpensive."

I thought you were a "PCA-GPX Chief Driving Instructor"? You do give breaking lessons to help keep racers out of this issue as much as possible, right?

On a street car "ice pedal" can happen with just hitting a bump in the road at the wrong time and confusing the ABS. I have never had a "ice pedal" issue in my car, ever, and I'm running smaller wheels and tires then my car came with from factory. Not by choice as someone swiped the Carrera Light 18" wheels off the car, probably one of the dealerships the car was at as I'm the 4th owner of the car.

People change the wheels and tire size on their trucks, SUV and cars all the time. Going from 18 up to 22" on trucks and SUV, some people will even do it on cars lol and never ever do you hear about people having "ice pedal" issues. Heck you can walk onto a dealership lot and they will sell you a customized Truck/SUV with way bigger tires then stock and the dealership won't break a sweat over it because it's just not a street issue. If it was, no dealership in the USA would sell you a vehicle setup like that as they would get sued for putting the people in danger and them crashing the vehicle. Way too much fear mongering with that post.

I can feel the difference. The car is more balanced through the corners when the front and rear tires are as close as can be to rotational speed. There is a lot less steering input needed. I'm not fighting the car around the corner, the balance is great. Maybe you should try it? Oh never mind on that, I see you have had a track alignment done to your car.

Squirrel :eek: