Why not measure from the floor to the center of the wheel, at proper pressure and fully laden?

Wouldn't that really be what matters? Not the measured diameter, not the listed diameter, but the actual match- or mismatch- in the radius (which defines the rotational speed).

I'll reiterate, but the rotations per mile is a better indicator for how much stress you will put on the diff...IMHO

A

 

You can get all the info. if different size include rotation per mile at the link below

http://www.1010tires.com/TireSizeCalculator.asp

 

Isn't that minimally different? It's not like a shift from 25 to 30 in sidewall.

 

So unknown sidewall stiffness is also a factor.

specially on a rear heavy TT. Maybe that's why 40 PSI rear and 36 psi front works so well.

I swear the car starts to develop a throttle off tail happiness very quickly when rear pressure gets below 39 PSI. Add some rear pressure and it's a totally different car.

 

Not true, and we already covered that earlier in this thread:


danger danger. ... Never, never use any online generic
'tire size calculator' if you are dealing with close tolerances, such
as trying to prevent rubbing or if you need identical front and rear
diameters like we do. They all mean well, and are simply designed
according to what the tire size nomenclature is *supposed* to
mean, but in the real world there is a wide range of variance
between what a tire is designated as and what it's real dimensions
are. For the real facts you have to go to the tire manufacturers
spec sheet. I have seen two different 245/45-16 tires differ in
diameter by 1/2 inch *from the same manufacturer*! They were
just different model tires. Recently one tire manufacturer stopped
making it's 305/30-18. Now they list a 315/30-18. *It's the same
tire*! They just decided that the actual tire dimensions were closer
to the 315 meaning, so after 6 years they just changed what was
printed on the sidewall.

To let you know how poor a ballpark answer they provide. Go to
tirerack.com and look at the spec sheets for 225-40-18 tires, and
you will find diameters varying between 24.8" and 25.5". The latter
value is taller than the average 235-40-18!

225-40-18 tires:

Hoosier A6 - 24.8"
Bridgestone expedia S-01 - 24.9"
...
Michelin Pilot Sport - 25.3"
Dunlop Sp Sport 01 - 25.5"

Now look at the supposedly larger 235-40-18 tires, eg:

Dunlop Direzza Z1 - 25.1"

I'm saying that however helpful these generic formulas are trying
to be, they are quite easily capable of being way wrong, precisely
when it matters the most to be right.
Joe

 

i hate it when people don't read the whole thread!
your the best joe!

 

It does change the implied sidewall height, but not by as much as from 25 to 30.
The point is that (as the rest of the post illustrates clearly) that tire vendors do not
make their tires the same size, and have no strict concerns about the accuracy of
the nominal size of the tire as printed on the sidewall. As the example shows, the
variance (whether across vendors or across tire models for one vendor) can be
wider than the gap between nominal sizes, such as between a 25 and 30 sidewall.

That's why http://www.1010tires.com/TireSizeCalculator.asp and all others like it
are useless/dangerous for our purposes.

 

Just read this entire thread- interesting, guess there is a good reason I prefer the stock wheels/tires besides aesthetics. I have had a few conversations ranging from not using the spare at all to using the spare only on the fronts because of the circumference difference. I have been in the unfortunate position of having to use the spare a couple times on the rear, about 50miles each time without any issues (that I know about). I believe the 997's don't have a spare from the factory which is also very interesting.

Has anyone had any front differential failure from using the spare?

 

it's not the wheels, it's the tires, but you are correct about the spare on the 997's. they have a can of fix a flat instead. The only reason has to be that the spare's diameter does not match the running tires.

 

I agree it's about the tire size, but I think there is probably more to the story. I checked my owners manual, says nothing about using spare on rear vs front. I dont know the circumference of the spare, but visually its obvious there is a significant difference. Perhaps when using the spare on one side it's ok as the normal tire is within spec, when changing the both rears or fronts with off-sizes that causes the size difference to burn out the viscous coupling - just a thought.

I am really interested to hear about anyone having a differential failure when using the spare. As you know, I've used the spare twice on the rear for extended periods and no issues. On both my situations, the sidewall was destroyed, the fix-a-flat wouldn't have done anything except make a ****ing mess.

 

think about the tires the way you think about a female body - it is the proportions that matter. If you run taller tires in back, the front tires need to be proportionally taller to match-up. Keep that in mind and you will never get into trouble. 996 cars like a 1:1 proportion front to rear tire height (36 bust: 36 hips, 34 bust : 34 hips) . The 997 does not - it is more of a .97:1 proportion (34 bust : 35 hips, 35 bust : 36 hips)

 

I was told that putting a different tire size on one side of a car is even worse than having both, because now your numbers are of on the same axle as well as front to back.

eclou, your comparison doesn't work for me. I like 40 bust and 36 hips

 

Re: 996 vs 997 spare.

On the 996 the front and rear are the same size (within 0.1")- so the diff is designed to have no inherent mismatch.

On the 997, the front and rear are the same size, and the diff is (likely) designed to accommodate these different sizes front and rear. Unfortunately, they would need two spares- one in the proper front diameter and one in the rear diameter. Easier to put a can of flat fix it in the trunk...

 

Don't quote me, but I think the 997 TT has an electronic clutch so it's really RWD unless the PSM electronics signal for front traction,

 

Then why get rid of the spare?