http://www.autozine.org/technical_sc...tion_4wd_2.htm

 

Radius of the tire when mounted to the wheel and on the car is actually a more important measurement than the diameter listed on the manufactures spec sheet.

First, the overall diameter is going to change based on the width wheel you mount it on.

The radius from the wheel centerline to the road is what needs to be considered. Think about what happens when you put drop the car to the ground. Pressure from the weight of the car on the tires changes the shape of the tire at the contact patch. The distance from the centerline to the road will not be the same as from the centerline to the top of the tire.

 

nice catch

 

Thanks

 

I've run different sizes because of the limited/zero availability of the OEM tire sizes, as well as because I have both 18" and 19" wheels. I thought the 19" wheels were causing problems even with a very small diameter difference until I put some brand new R888 track tires on OEM wheels and guess what: Still having PSM/CEL issues!

There's way too much heresay out there on this topic, plus unless people are actually measuring their tires to see what the real size is, this whole discussion is pretty much useless IMHO.

P.S. In my particular case I'm leaning towards a bad ABS sensor as the root of my CELs, but haven't had a chance to replace any of them to see if that fixes it.

 

Like I have said on previous posts and had a thread about it in the past...
I have ran a .5'' difference now for the around 25,000 miles, numerous dyno pulls, long road trips and many 175+ mph pulls with Zero issues,, I get my car checked by Porsche techs when it in the shop. They have told me that will not cause a failure Now, like they were lead to believe in the past also?
Also don't go by published OD because they usually are not measured on our size rims.. Wider rim smaller OD,, narrower rim larger OD.. I have measured the tires mounted on our stock rims.. I have posted all this stuff in the past.
Just giving you my experience.

 

Fair enough, but no one is measuring their tire, radius, diameter, or circumference once it's mounted, so the initial mfg numbers are the next best option.

True, but then you then have to factor in tire pressure and no one is doing this. Again, the mfg numbers are the next best option.

Again, unless someone is using an obvious wrong set-up, whether that be a mismatch between front and rear, or even mounting to an incorrect rim size, it's highly improbable that minor percentage differences in the tires radius/diameter/circumference will mean anything.

Over 250 posts and not one person can offer any evidence to say otherwise. I know there are a lot of smart Porsche owners, but no one knows.

 

brethren,
i am going to offer you all a real world solution, for your imaginary problems.

ready ?
245 35 19 front
305 30 19 rear.

 

Stop it. We all know it 235 40 18, 315 30 18. Only those who think they can combat oversteer with tires alone go to 245s.

 

Which is precisely why you should use "revs per mile"... and specifically the differences in revs/mile for each tire choice you are evaluating

 

265/35-18 works too....

 

Come on guys. All this talk about diameter and radius and now it's RPM. How do you think RPM is calculated?

A= diameter (converted to feet) times Pi (or Pi time radius squared for those who think radius matters most). This gives you circumference. Then take 5280 feet (distance of one mile) and divide by A and magically you get RPM.

You may not get the exact number shown by manufacture but it it'll be real close. Remember, whatever percent difference you have at RPM will be the same for diameter or radius.

Don't forget to account for your rim size an tire pressure and sprinkle you magic Porsche fairy dust.

 

Is that one of those dealer only items?

 

Wow... LOL...
Also, What do you call real close? I have seen and measured tires on many rims.. A .5'' difference in rim width usually is about .25 '' to .30'' difference in diameter.. Also seen tire specs vary from published given specs.
Remember we are dealing here With Porsche people,, that need and want to be to the exact specs within the box.. LOL.

 

You mean radius X 2 (not squared). Actually, this is absolutely true if the tire were a rigid body. However, if you check the manufacturer's specs, you will see that rev per mile does differ even for the same diameters.

An explanation from TireRack states:

"Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire’s published overall diameter (which would only reflect the tire’s unloaded radius), calculating the tire’s absolute rolling circumference isn’t possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate."

What I'm more interested in finding out is the design of the differential that we're all concerned about. A differential is suppose to account for the differences in rotational speed and in most cars is working almost ALL the time (unless you only drive in a straight line ). Is the center differential for the AWD cars really designed to handle less than the front and rear differentials? Just curious....

P.S. The Porsche fairy dust is the most important component .