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Old 03-31-2013, 06:12 AM   #1
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Cheap Performance Mod... Or is it?

As nothing more than a thought experiment, I was wondering what would happen if one was to reduce the total wheel/tyre diameter, ie. replacing rear 265/35 tyres with 265/30 or even losing an inch in wheel size and not compensating with a higher profile tyre?

This would reduce the angular inertia of the wheels considerably as mass is being removed from the outermost part of the wheel.

It would also have the same effect as running a shorter rear diff... no need to explain this benefit to your average "Larry Leadfoot".

It would also allow for more clearance in the wheel well so more caster could be run (better suspension setup?? and increase in wheelbase), as well as lowering the centre of gravity a little.

Now the question is, what am I missing here? I really would like to be strongly critiqued on this one, as it seems that there is a lot to be gained with very little lost.

And yes, I realise that the speedometer will be off and potential ABS problems etc... Not really interested in hearing these tired old answers, want to know about negative effects in handling or other performance issues and why they will come about.

Thanks in advance.

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Old 03-31-2013, 06:58 AM   #2
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If nothing else, it will look like a**.
Don't worry I've got the microfilm.
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Old 03-31-2013, 08:15 AM   #3
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Lower gear ratio, lower top speed, better torque out of corners, higher revs on the fwy. If you AX it often means needing to shift to 3rd gear- not a benefit. Some tracks will be geared better, some worse. Ice pedal from an ABS mismatch is no laughing matter.

Several Boxster drivers have experimented with this. Most top drivers choose standard diameters. Try it and report back with your impressions.
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Old 03-31-2013, 11:01 AM   #4
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Good question & another reason I prefer to buy a car in need of tires instead of a car with "good" tires. As you may suspect Porsche gives some thought to details like this also & another reason to think twice before altering Porsche engineering for which we all pay so dearly for. Really no alternate sizes to accomplish what you want & don't forget going to a shorter than stock size racks up miles on the odometer faster than actual. 255/35/18 is a bit short so you lose load carrying capacity & have to increase air psi. XL sidewalls would help if available. This might be worth it for the small engine Boxs. 285/30/18 is the best option & gives noticeably quicker acceleration thru the gears, but requires wider wheels, increases understeer. I changed to the 285s when I changed to the 3.6L engine for the traction but it just was not worth the drawbacks.

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