Spare Tire Replacement

[DANNOV]

I do some long trips in my 03 Boxster S. I have the stock 205/50-17 fronts and 255/40-17 rears. I have stock Bridgestone SO2-A tires. The diameter of the 205 is 25.2" and the 255 is 24.8", this is data from the Tire Rack site.

I am not comfortable with the stock 105/95 17 temporary spare tire.

Has anyone attempted to use a 205/50-17 as a spare tire in place of the dinky temporary spare provided by Porsche?

I'm thinking that perhaps I can carry a 205/50 17 as a spare. Its diameter is slightly bigger than the normal 255 rear (25.2" vs 24.8") but in an emergency it probably won't make that much of a difference if put on the rear. Or, will it? In the worst case it could confuse the PSM system. Could extended driving with such a misfit tire put an overload on the PSM system?

Also, will a full sized 205 fit where the spare tire sits? The spare is a 105, about 4" in width. The 205 is about 8.0" or twice the width. When I eyeball it, it seems to fit. Has anyone ever done this? I'm willing to give up a bit of trunk space for a real spare tire.

When traveling, if I have a problem, I just want to put on the spare and feel that I can drive safely at 65 mph for as long as it takes (perhaps days) to get a repair.

Also, 205 is the narrowest 17" tire I could find, and there are a lot of choices. Does anyone make a 17" narrower than 205? I can't find one on the Tire Rack site. Any ideas about a cheap 17" wheel? Tire Rack has one for about $200. Are there any cheapo wheels that would work?

[adsach]

When traveling, if I have a problem, I just want to put on the spare and feel that I can drive safely at 65 mph for as long as it takes (perhaps days) to get a repair.

Travel for days? I wouldn't travel for days on a spare tire - that means you are travelling for days without a spare tire available if you need one. That doesn't fit into my view of wise travelling.

For freeway cruising at legal speed, the limited speed tire does fine.

It is a limited speed tire, not a limited distance tire, so until the tread gets low you can keep driving on it.

Overload PSM? IMHO no. PSM combines Yaw sensor, wheel speed sensor, and steering input to make a decision. Spinning wheels that are within a few MPH of each other are not a problem requiring PSM intervention. I have had the limited tire on for 30-40 miles of backroads without PSM complaining. Of course I was driving like I had a rear tire 3.5 inches wide on one side and 10.5 inches wide on the other.

Fit in the front trunk? Yes.

Fit in the spare tire position? not likely. The smallest I have tried is 225x17 and it fits at an angle, basically using up the front trunk. A rear tire won't even fit in the trunk, unless you have only strips of rubber left on the wheel.

One other thought: I have heard that Porsche calculates the spare tire into the crumple zone calculations in case of a major collision. If I were you, I'd accept the spare tire as it is and be happy I got a car with a spare tire rather than a tube of sealant and an air pump.

Ad Sach

[DANNOV]

adsach

the skinny spare has a max speed of 50 mph per Porsche, not the "legal speed" you mentioned. To be safe, I would probably run under that 50 mph speed. If I had a simple flat my first inclination would be to repair it on the spot with a plug, using the spare to only get me to some safe spot for repair.

My big concern is a cut tire or a blowout that cannot be repaired. I had a bad experience in central Nebraska trying to find a replacement for a blown out 17" tire on my wife's 95 Vette. When you get away from major population centers it can be very difficult to find a 285/40-17 tire, or even a reasonable substitute. I had to limp along for several hours at under 50 mph to find a replacement. The only tire I could find was some kind of 17" truck tire, which I soon discovered was of a different rolling diameter than the stock Vette tire. This caused the Vette's traction control to go nuts because it was detecting a wheel that was rolling at a different rpm than the other three That problem was solved by shutting down the TC. But the car was impossible to drive as it had a slight lean that made it unstable at any reasonable highway speed. It took me another two days (one day was a Sunday and nothing was open) to find a proper sized tire from a Chevy dealer. This was a real PITA experience and destroyed my vacation.

Last fall I was driving my Boxster up in the upper peninsula of Northern Michigan. I was a bit haunted about what I would do if I destroyed another tire or if I broke down way out in the northwoods. That trip was 1200 miles and I saw one Porsche when I left Chicago and did not see another for 1000 miles! When you get away from the big cities; help, parts and tires for these cars can be hard to find. The alternative is to have it hauled to the nearest Porsche dealer and continue via rental car. A real spare tire for the Boxster would be nice. I don't want to relive another bad experience.

[her_986]

From what you have said above, it seems to me that you carry some kind of inflator, correct?? If so, have you looked into the MY03 base. I have a MY03 base, however it has the ‘S’ standard wheels(17’s), can not remember if standard ‘03 are 16’s. But, is not older boxster 16’s?? If so, you could put the smaller tire in the front deflated reducing its diameter!!! A lot of owners trade up on wheels, perhaps there are what I think are called ‘Take Off’s’. PSM: I do not have it, but you would think that the PSM system was designed with the knowledge that, some day it would have to deal with the spare on the rear of the car, no!! Just some $.02 worth… :unsure:

[DANNOV]

her 986

Interesting idea, but a Boxster S has larger diameter brakes than the normal variety. I doubt if a 16" wheel could fit, especially on the front. Also, I have looked at the specs for various 16" tires and the diameter seems to be an inch or more smaller than my existing tires, i.e. the diameter varies with the aspect ratio. What I really need is a 17" tire with about a 185 width and 25" diameter, mounted on a 6" rim. That would probably be narrow enough to fit in the trunk (3" wider than the skinny spare) and the diameter is close enough to the existing tires. Unfortunately, no one seems to make a 17" tire less than 205. I'm beginning to think that the skinny spare is probably my only alternative. Guess I'll just have to live with it.

[adsach]

How about a Motorcycle tire?

The stock spare is a 105/95-17 which is pretty close to 110/90-17 in a motorcycle size. The contact patch on modern motorcycle tires is less than it used to be, but some of the weight ratungs go to 992lbs. I wouldn't want to go aggressively cornering.

The ability of a motorcycle to disappate heat under such a heavy load might be an issue. A high speed tire might be able to handle the heat.

I don't know how motorcycle beads compare to the spare tire wheel, but at $40 for a motorcycle tire, it might be worth the experiment.

Ad Sach

PS> Legal disclaimer;) I NEVER recommended it, only suggested it as 'an idea'

;) ;)