Let me just just say first off that I am a complete tire fitment imbecile.
I guess I just have no idea what the different dimensions actually mean.

I am getting some HRE P40's:
19x8.5 + 19x10.5

I want to stick with PS2. But am concerned with keeping the same rolling diameter on my new 19" tires as stock - to keep the speedo, PSC and ABS working correctly (are there any other reasons?). I read a while back that Ken recommended keeping it stock.

So... what are my options?

The shop I am going through want to put on:
M1 235/35-19
M1 295/30-19

235 is the circumference right? front is the same than, back is different.
Surely Michelin make tires wide enough to fit 19x10.5 with the same circumference?

Advise is much appreciated!

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The first number represents the width of the tire. The second is the percentage of tire sidewall height relative to the width. The last is the wheel diameter.

So your stock 265 is thinner than the 295 suggested but the 30 is smaller than the stock 35 which works out to almost a wash because the stock is thinner.

Here is a simple calculator to help you. Tire Size Calculator - tire & wheel plus sizing

Stick to 235/35 fronts and 265/35 rears. 10.5 is in the recommended width range for the 265mm N-spec PS2 and so why not keep it all stock. 295 rears with 235 fronts will lead to understeer.

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- Suneet


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thanks guys! I am finally starting to understand how the numbers fit together.

I have been reading up from other Cayman owners that have these wheels and everyone else I am finding have 19x11 rears.... In that case I will probably get 305/30ZR19 tires

... more investigation first.

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Careful with 19x11 and 305 tires. Your wheel offset needs to be perfect for it not to rub on the fenders or the suspension.

Yeah... just got off the phone with HRE.

19x10.5 is the correct size for the cayman.
Only reason they were fitting 19x11 before is that they did not make 19x10.5.

So I'll keep the stock 265/35. Everything will jazz nicely. Like this banana ->

Edit: Banana seams to be broken - is this a sign of things to come? Only time will tell

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Okay so I am being told that the ideal tire for the 19x10.5 is the 295/30. Its true - but is it worth changing from stock width for the 'ideal' if the 265/35's could also do the job.

Shop is telling me that if I go for the 265/35's the tires will be concaved in the center and the rims of the wheels will be more vulnerable to damage. Is this true?

Thoughts?

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Very possible. There is a member here that posted a picture of a 10.5 or maybe even an 11" wheel with a stock 265 tire. It looked horrible as the tire was stretched.

I would go safe with a 295 or even 285. The wider you go the better rear grip you will get. Good for the 1/4 mile but has an affect on the steering of the car. Since the rears grip more, it can cause understeer. You will only notice this if you track your car or drive nuts on the road.

Mine is a street machine so when I got Bilstein PSS9's I set the rear firmer compared to the front which should counteract any understeer my 15mm spacers caused in the back.

If you ask, the spacers in the rear bring my stock wheels out so they are closer to the fenders (visual look). You won't have to worry about this with aftermarket as they build to a spec that looks close already.

The 265/35ZR19 has a wheel width of 9.5" and tire width of 10.7", this is the standard size for a Cayman.

Tire Rack PS2 Specs

It's true that the tire has an extra lip to help protect from wheel damage and this will not be very useful for protecting your wheel with a 265mm wide tire. But honesly low profile tires aren't good for this anyways... plus I'm sure you're gonna be pretty careful with your new HRE's. As far as looks go...I don't think you will mind the look as it's a very minor stretch.

265mm = 10.4 inches, and tread depth is actually 10.7'' so not a great amount of stretch at all. I would go with 265's because of the weight savings over 295 and to keep the cars staggered balance. Or go with 275 rear and 245 front. I wouldn't go with 285 unless you use 255 fronts. And as you keep going wider, the weight will generally increase. I would at least see what 235/265 looks like... I doubt they will look concave in the middle.

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- Suneet


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If he did go with the 295 rears would you step up the fronts from the 235 stock size?

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Current Cars:

2008 Orange - Limited Edition Boxster S
2004 Atlas Grey - Boxster S

Personally I would... the biggest fronts anyone has run on here are 255/35/19, that's probably what I'd go with. But I guess it depends on what you will be doing with the car, if its primarily a daily driver you wouldn't notice the understeer from using 295 in the rear and 235 in the front...

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- Suneet


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I would say that you could go wider on the rear but the car is going to understeer more. I have seen people go with 245/35R19 front and 275/35R19 rear without any problems. I have seen guys use 285's on the back though.
If I can help with the tires let me know.

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Jim Holloman | Sales Specialist
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The Tire Rack
7101 Vorden Parkway
South Bend, IN 46628
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