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Discussion Starter · #1 · (Edited)
Would this be possible? The tires would have higher diameter, but the car has stock, not lowered suspension.

if so, I assume you'd need to recalibrate speedometer (not sure who does that...think my car as it is thinks it's going 3 mph faster than it is on 20 inch wheels)? Would it work on 20 inch carrera s wheels or what wheel size would be needed? Or you need spacers?

Think I have a typo in title...goal is to upside the tires a bit and adding +10/0-0 would increase both the width and diameter...so maybe +10/+5-0 isn't needed. Tires are offered with either combination though and curious if it is possible to do...
 

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There are numerous threads regarding upsizing, offsets, spacers, etc. They are applicable across models. Please search.
 

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Would this be possible? The tires would have higher diameter, but the car has stock, not lowered suspension.

if so, I assume you'd need to recalibrate speedometer (not sure who does that...think my car as it is thinks it's going 3 mph faster than it is on 20 inch wheels)? Would it work on 20 inch carrera s wheels or what wheel size would be needed? Or you need spacers?

Think I have a typo in title...goal is to upside the tires a bit and adding +10/0-0 would increase both the width and diameter...so maybe +10/+5-0 isn't needed. Tires are offered with either combination though and curious if it is possible to do...
Hi minthral,

If not already aware see the links. Such obviously do not answer any body/frame clearance issues, but the info therein does allow one to compare tire to tire issues and calcs the speedometer factor, etc.

When required :( I am planning on +1 sizing the tires on LG from 235 to 245 and 265 to 275. All still staying at 35 x 20 of course.

Tire Size Calculator - Compare Two Sizes and Tire Size Calculator - Tire & Wheel Plus Sizing

:cheers:
 
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Discussion Starter · #4 ·
Excellent tools. It seems going to 245/275 is pretty safe and easy. Only thing that bothers me is the speedometer. I can't find any info on adjusting it or how to calibrate. Going to dealer would probably give me a blank stare 'that's the wrong tire size.' I'm sure as it is mine is 3 over while the car is completely stock.
 

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I never bother calibrating the speedo. I have a calibration in my head I apply to the speedo as standard, or use the speed on my GPS instead.

Usually speedos read high anyway, so putting larger tires on tends to correct this. I use 275/35R-19 and 245/40R-19 as my track/autocross tires, the speedo is dead on with those. You're going a bit bigger so your speedo will probably read a little low. I have the sport suspension (20mm lower) on mine which fit without issue, so your fronts will probably fit fine, your rears are a bit big, 2" larger diameter.

I don't know if the speedo reads the front or rear tires.
 

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I'm running MPSS 245/35-20 and 285/30-20 tires on the stock Carrera S wheels for my Cayman S. Fits with no issues.
 

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I wouldn't really bother if the circumference is within 1% of the original size. Anything more, you probably should be using those tires. The traction control etc start to go silly when the geometry chances too much.
 

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Discussion Starter · #8 · (Edited)
I'm running MPSS 245/35-20 and 285/30-20 tires on the stock Carrera S wheels for my Cayman S. Fits with no issues.
Great info. Surprising 285 fits on the back... thats 3/4 inch wider than the stock 265. I'm wondering how much wider can you go?

Front wheels are 8 inches wide with 9.25 inch wide tires (235). Rear wheels are 9.5 inches wide with with 10.43 wide tires (265). The delta in difference in stock wheel to stock tire size is +1.25 for fronts and +.93 for rears. Seems the rears do have more room to be wider. If you size up the rears to 285, the delta from stock is now +1.72 (only .47 wider than stock front wheels).

Since 245 (9.65 inch wide tires) works on the stock front wheels, then the actual delta can be as high as +1.65 for the front tires.

So is +1.75 inch tire size difference than wheel the max maybe (+.50 inch tire width over stock, which is 1.25, but going up to 1.75 works)? Or can you go higher?
 

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Discussion Starter · #9 ·
Anyone know if brakes on this car would become a problem if diameter is 5-6%+ from stock? The tool has a warning that it shouldnt be higher than 3%.

Would be great to know how to re-calibrate speedometer too...

Also, what would be consequence of getting rear wheels the same size as front?
 

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Discussion Starter · #10 · (Edited)
I did a test and my 2014 boxster actual speed is about 2-3 mph slower than what the speedometer says (20 inch carrera s wheels and p zero tires) at 70 mph (actual is 68ish). If I were to get +10/+5 tires, that would actually fix my speedometer (bigger wheels make speedometer show your speed less than what it is).

As expected, dealer said "can't help you if you dont use stock wheels and porsche approved tires" in regards to speedometer calibration.
 

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Discussion Starter · #12 ·
It is not just a spedo issue. It would seem to me that the odometer is not going to be accurate.
Am I missing something?
Odometer and speedometer get the same reading from my understanding. both are probably from the rear wheels spinning and an equation that calculates wheel diameter. Unless someone knows beter...
 

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I wouldn't really bother if the circumference is within 1% of the original size. Anything more, you probably should be using those tires. The traction control etc start to go silly when the geometry chances too much.
you may wanna look in that spring pirch...on the stock techno wheels it awfully close...


lemon
 

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Discussion Starter · #15 ·
P Zeros in the back are near shot after 12k miles...far outside corners show wear 'going down to the next rubber layer,' but the inside seems okay. These tires are horrible in anything wet after wear - they were bad enough new. Though they are good dry cornering tires, I'd never buy them again for anything else...

I purchased a set of 245 40 20 and 275 40 20 Continental DW tires after research on wanting the best for daily driving (no track). Local Indy shop has them and I hopefully will have them installed next week. They are less than $200 per tire (2 times cheaper than P Zeros or Michelin Cup). There are some other tires I could have tried, but they had too many 'unknowns' but Continental DW are very well established.
 

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P Zeros in the back are near shot after 12k miles...far outside corners show wear 'going down to the next rubber layer,' but the inside seems okay. These tires are horrible in anything wet after wear - they were bad enough new. Though they are good dry cornering tires, I'd never buy them again for anything else...

I purchased a set of 245 40 20 and 275 40 20 Continental DW tires after research on wanting the best for daily driving (no track). Local Indy shop has them and I hopefully will have them installed next week. They are less than $200 per tire (2 times cheaper than P Zeros or Michelin Cup). There are some other tires I could have tried, but they had too many 'unknowns' but Continental DW are very well established.
12k in the back for high performance tires on a sports car is about right. The stickier the tires the shorter the tire life is going to be. If you're worried about treadlife, you could buy some Prius tires and they'd last almost forever. IMO, complaining about tire life on a high performance sports car is about like complaining about fuel economy on a high performance sports car. It's kind of missing the point.

Secondly (to the thread), isn't a 40 series sidewall too tall given the diameter and section width? Stock is 235/35/20 and 265/35/20, so if you go up 10mm in width, at very least you should keep the height ratio constant and not go up in that as well. The tire is going to be taller which is not only going to mess up your speedometer, but you're also going to give the call taller effective gearing - which makes it slower.
 
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Discussion Starter · #17 · (Edited)
12k in the back for high performance tires on a sports car is about right. The stickier the tires the shorter the tire life is going to be. If you're worried about treadlife, you could buy some Prius tires and they'd last almost forever. IMO, complaining about tire life on a high performance sports car is about like complaining about fuel economy on a high performance sports car. It's kind of missing the point.

Secondly (to the thread), isn't a 40 series sidewall too tall given the diameter and section width? Stock is 235/35/20 and 265/35/20, so if you go up 10mm in width, at very least you should keep the height ratio constant and not go up in that as well. The tire is going to be taller which is not only going to mess up your speedometer, but you're also going to give the call taller effective gearing - which makes it slower.
I realize how long p zeros are supposed to last and not complaining about that. My complaints about the tires are wet performance and noise.

A 40 tire is .4 inch wider on each side (less than 1 inch total). As mentioned, on stock everything and p zeros, speedometer reads I am going about 3 faster than I really am at 60 mph. It is normal for speedometers to do this is my understanding (or to be inaccurate anyway). By increasing the wheel diameter, I am actually correcting the speedometer. Not sure about the effect on gearing, but don't think the size change is significant.

Also, this is my daily driver and I don't want a pure raw sports car for that. I bought the car for a mix of luxury and refinement...not a track experience. Comfort, noise, tread life, MPG, and etc are parameters of a car, so being blindsided "it's a sports car those things don't matter" is IMO missing the point.
 

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Discussion Starter · #18 · (Edited)
I put these tires on and have barely had a chance to drive on them so far, but initial impressions:

- Night and day quieter than p zeros. Going down a rough road, there's literally almost no tire noise. Granted DWs are new and P Zeros rears were near dead.
- P Zeros had tighter cornering and better high speed stability. I may need to break in the DWs to do a real test (they still got markers on the treads)...but so far it feels the DWs slip slightly and quickly correct, but P Zeros don't slip in the same situation.
- Noticeable comfort increase (I can feel the tires giggle/ hear pounding as large impacts are absorbed, instead of suspension crashing)
- DWs don't seem to pick up dirt and flick it into wheel wells like the P Zeros. Not sure why but I dont I hear the "clicking ticking" noise on the same road.
- Wider higher tire feels safer. Probably wouldn't scrap the wheel if I rubbed a curb.
- Gap decrease around wheels looks better IMO
- 20 inch wheels look pleasantly smaller with bigger tires on them IMO
- Speedometer reads 1-2 slower than I am going now
- No weird tire noise on doing sharp slow turns
 

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The old rule of thumb for rim VS tire width is 90%-95% for the max contact patch.
265x 90%=238.5mm=9.38"--95%=251mm=9.91" width rim...
285x90%=256.5mm=10.09 --95%=270mm=10.65 rim width..
carl
 

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Discussion Starter · #20 ·
I've driven about 600 miles on the tires now and here are my observations:

- Noise increased somewhat, but still considerably quieter than P Zeros. Unlike P Zeros, DW tire tolerate rough roads with minimum noise.
- Ride softness firmed up a bit, but still more comfortable than P Zeros. Prior to break in, felt like the car was on tip toes, but now is walking on all feet. Definitely still a sports suspension ride, but with a little padding.
- Traction increased a great deal. I am able to do the same thing I did with P Zeros, but seems these tires handle different... they are like suction cups at lower/mid speed, but higher speed corner leads to predictable progressive break away (P Zeros are like suction cups at higher speeds then have a quick break away/ traction loss at the limit).
- Steering response is really relaxed down the center and the car goes straight without correction. P Zeros were sensitive to the road and needed lots of adjustment. Doing steering maneuvers, DW tires really open up with more response, but are more numb with minimum response; I like this for the streets because I don't like having to correct a lot due to bad roads.
- First fill up I measured MPG has increased by 1-2 despite having bigger tires and I drove a bit harder than normal. Weather is warmer, so maybe need more tests. I notice my Odometer and Speedometer is 3% under reporting and I manually make the calculation (+3% miles / gallons to full); oddly, MPG reading is more accurate now (it was over reporting before). I also notice the tires have very little rolling resistance...going down the same hill in coast mode, P Zeros would slow down, but the DW tires speed up.
- No idea how to explain this, but car feels faster...as if it has more torque. Feels more responsive to less pedal...as if sports mode is enabled, but its not. I've done my 0-60 test in the same spot, and it does seem to go faster for some reason. It seems like traction control, though enabled, is holding the car back less. Maybe this is due to new wider tires with full thread... not sure.

Overall, I am very pleased with Continental ExtremeContact DW tires so far and definitely think they are an improvement over P Zeros. Its amazing how much of a difference a set of tires makes... really changes how the car feels and drives.
 
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