How do you estimate tire sizes and widths?
 

This last week I ordered a set of Avon Tech R's from Tirerack after checking the Section Widths (SW) and Tread Widths (TW) to make sure they would be at least the same size if not smaller than the Kumho Victoracers I had before that fit (just barely). Sizes are 225/17/45 front on a 17x8 Fuch replica and 275/17/40 rear on a 17 x 10 wheel.

The measurements on the rears (where the problem lies) Kumho were 11.1 SW and 10.3 TW. The measurement on the Avons were 11.0 SW and 10.2 TW. This appears that they would work.

When I had the tires mounted, the rears were at least 1/4" if not more wider than the Kumhos they replaced. Consequently I had to use a set of 5 yr old street tires at the track event I had planned for Saturday--peeved at myself for not giving myself more time for surprises such as this.

So how can I order a set of different brand track tires if the stated widths are not what they say they are? Is there an actual measurement somewhere on the brands/sizes?

The section widths are always specified for a given wheel width. The section widths specified for each manufacturer may have been for different width wheels. A good rule of thumb for conversions is 1/2 inch change in section width for every 1 inch change in wheel width. Of course, the 1/2 inch change in section width is shared equally on the inside and outside sidewalls, so you should be able to predict potential interference.

and that makes sense Howard but the Kumhos and Avons were mounted on the same wheel (after the Kumhos wore out I wanted to try a different tire) front and rear so I would have expected similar changes in section width--however the Avons ended up significantly wider for some reason.

That is a HUGE pet peeve of mine.
In this day and age there is no reason why tire companies aren't held to quality standards like every other automotive manufacturer.

12" should be 12". If it isn't your getting ripped off. Years ago I purchased a set of Pirelli 345/35-15s for my 12" Fuchs. When they wore out I purchased a set of less expensive Yokohamas. Same size. The Yokos were 3/4" wider than the Pirellis! I later figured out that ALL Pirellis lie about thier width and are actually smaller. I guess it's the big dick thing. Their ruler isn't the same as mine.

So to answer your question, there is no way to know what you are getting. You have to ask around and find people who have had similar experiences as my own (and now yours).

That does suck. One thing that might be worth looking into, however, is the measuring rim width. The section and tread widths may change for a given tire size when measured on different width wheels. Also, it might matter what one tire manufacturer considers as the optimum wheel width for a given tire size, and would therefore publish section/tread dims based on that particular rim width. I would try to look-up each tire manufacturer's site and see what rim width they used to measure. Tire rack may do their own measuring (doubt it, not for every size, I can't imagine how much of an effort that would be). My $0.02

-Garen (87 930 CS)

It IS annoying as hell, but it's not really a quality issue, and probably not deliberately misleading marketing -- the simple fact is that there is no industry-standardized practice for how the measurements are made. Each manufacture is free to use their own methodology and whatever technique seems appropriate.

Well, I did some research, partly to test my own theory. Sure enough, the "measured rim" size is the problem. If you look on Tire Rack's web site at the specs for each tire, you will find that the Avon has a section width of 11 inches on a 8" rim; the Kumho has a 11.1 inch section width on a 9.5" rim. If you bring the Avon up to an 9.5" rim you end up with a section width of about 11.75 (11 + 1/2 X 1.5" difference in measured rim size). Thus, the Avon is wider than the Kumho for a given rim size. For a 10 inch rim, like yours, both tire section widths are about a 1/4 inch wider than the width on a 9.5 inch rim. Also note that the Avon is not recommended for a rim as wide as 10 inches.

Howard is absolutlely correct. The measuring rim is critical to determining the section width and height of a given tire,

I use a slightly different rule of thumb <FONT FACE="SYMBOL">D</FONT>.1" section width for <FONT FACE="SYMBOL">D</FONT>.5" wheel width or .2"/1". As I said only slightly different.

I had extensive notes from years ago when I was fooling w/ 7, 8 & 9" Fuchs and 225/50 and 245/45 tires. Unfortunaly I couldn't find them this morning.

Another feature affected is the tire height, a wider wheel will not only stretch the section width out but also shorten the sidewall height.

note that the tread width is not affected and does not change w/ diferent wheel widths

That's all good info but does not directly address the issue.
What we are talking about here is different actual measurements of two different brands of IDENTICAL tire sizes on the SAME rim.

The manufacturers are not made to conform to a set standard for measurement of either tread width or section width of a given tire on a given rim. Fitment data given by the manufacturers only confuses the issue due to no standard rim construction or size used with a specific tire size.

Therefore there is no way to know exactly what you are getting without actually mounting the two different same-sized tires on the one rim yourself.

Good information guys thank you. Looks like its my fault for hastily assuming the size was the same base on the measurements given (however I ignored the rim size effects). Lesson learned.

I think there is an industry standard that specifies an allowable range of measurements for a given standard size. Some manufacturers stay on the high side of the range, others stay mid-range. Some manufacturers vary within the range depending on the type of tire (anybody remember how much wider Yoko 008RS's were than 008's?). In any case, the spec sheet provided by all manufacturers for each tire size will tell you exactly (or nearly so) what you will get. Just don't forget to compensate for the size of the rim on which the tire was measured.

Yeah, thanks, that rings a bell. I was too hasty when I said there was no standard. It's just that the standard isn't very precise for resolving tight-clearance fitment issues.
John

A standard that will allow 3/4" variance on 345mm is useless.

Hey Jon I can't keep up with you now what the heck am I gonna do if you actually show up at the track with good tires.