I need to get a good set of track tires. Durability and price are as important to me as adhesion. I was thinking of getting the Toyo RA1's.

Anyways, checking on some web sites I found that the diameters for the tires were as follows:

 

First recognize the calculators are theoretical and can be off significantly from the actual tires. Even so, those sizes are too different IMHO:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
305/35-18 4.2in 13.2in 26.4in 83.0in 764 0.0%
245/40-18 3.9in 12.9in 25.7in 80.8in 784 -2.6%

Not sure about the effect on PSM, but handling and wear could be unpredictable.

 

Me thinks we have an engineer in the crowd.

 

The actual specs are aways to be found on the manufacturer's
spec sheet:

[media]http://marktg.toyotires.com/file/18968.pdf[/media]

The diameters are 26.3" and 25.6". I wouldn't
run that big a difference myself.

 

Here's a repeat of a post I made about using
any of those online tire size calculators:

Wross996tt alluded to the issue. Never, never use any online
'tire size calculator' if you are dealing with close tolerances,
such as trying to prevent rubbing, or if you need exact
diameters. They all mean well, and are simply designed
according to what the tire size nomenclarure 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.

Joe Weinstein

 

Hey Guys-

What diameter difference should one consider as a 'maximum allowable'??

Stock tires are 25.1" and 25.2", so 0.1" 'mismatch"

Commonly used 235/ 40-18 and 315/30-18 are 25.4 and 25.5 respectively... same 0.1" differential

Using a front 245/40-18 is 25.7.... only 0.2" difference (with the 315/30).

Comments, thoughts?

A

 

My experiance is that the rear tires wear at a higher rate than the fronts, about 2:1. The rear tires are already smaller than the fronts in diameter. So if you caculate the difference when you have burned thru the rear tire 1/2 the wear value of 9/32's of an inch, it even get worse.

 

On Turbo cars DO NOT use tires diameters that are more off than .5 inches. You'll fry the viscous coupling in the center differential.

 

I won't go more than .3" diff, and I much prefer the fronts to be a little
bigger.

 

Just as well. I really wanted 235's up front and 315's in the rear and the Toyo's don't come in either. I guess I'm going with the Pilot Sport Cups. If its good enough for the RS...

It is funny that the Toyo's would have such a big discrepancy within a single line of tires.

 

I have Nittos NTO1s on my stockers 235f &315r and I'm very happy with them! should give them a try!

 

If it will fit your car, the 335 RA-1 is a much closer match to the 245 front. A few people on this board have posted those as the sizes they are running on their 996 tt's. I have ordered a set or those particular RA-1's for some new wheels I'm having made.

I also run the Nitto's mentioned above on my stock tt wheels and I really like the tires. Don't want to get caught in the rain, though.

 

Wow. Those are monster tires! What size wheels are you ordering? What are the offsets? Let me know how that works out.

Anyway, I decided to go with the Nitto's. Saved about $350. Like you said; if I get caught in the rain on the way to the track, I'm screwed.