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Why do slick tyres have a different numeric sizing ? And how would someone go choosing the correct size ? For example if on a normal passenger vehicle we have a size of 205/50/15 would a slick tyre of 205/550/15 or something in those lines be similar ?

Thank you

Hey Dimitrios

Have a look at this link: https://merchandising.demon-tweeks.com/DT/product-images/size-guide/motorsport/tyres/toyo-sizeguide-rs1.jpg

It says here that the middle number is the diameter of the tire, so the length from side to side of the tire + rim (in mm). I'm actually not sure why that is...

There's a calculator here for reference: https://www.cargister.com/calculator-race-tires.html


As Andre said, the wheel size for race tyres will have the outside diameter, rather than the aspect ratio.

The reason is actually quite simple - with the conventional, road, tyre size designation the overall diameter will change with the width and/or the aspect ratio - and, of course, the wheel diameter. Worse, there can be quite a big variation between different tyre companies' sizing of their tyres - I've seen close to an inch for 'identically' sized tyres.

With a race car that is introducing another variable, as that change in diameter will affect the overall gear ratios - like changing the diff' ratio every time you change tyre width. There may also be body or ground clearance issues - the latter could be quite significant for aero'.

Unfortunately the links above do not work , or at least correctly . I type in the examples I have given above but it keeps on showing me the initail page and 17" rims . I am still trying to understand the similarities so I can choose a set to try and hit the track and if possible some hill climps .

Ahh... yes I see your problem here, that link isn't particularly helpful if the page doesn't work!

Lets see if this one is any better https://tiresize.com/tyre-size-calculator/

This link will give you the estimated wheel sizes, you can then have a pretty good idea at what kind of size you'll be looking at.


Did you read my post?

Your 205/50-15 tyre size is a nominal width (the first number, which will be with it fitted to a specific width wheel, that can be different between brands) and aspect ratio ( the second number which is the ratio between the sidewall height to the tread width) - the last number is the wheel diameter it is to be fitted to, and that is held to a very tight olernce as too loose and the tyre will slip on the rim and, probably, leak out air, too tight and it will be difficult, or impossible to fit it over the safety beads on the rim.

So, while the nominal diameter would be 23.07" could in practice be ~195-215 mm width and ~45-55% aspect ratio, depending on tolerances and marketing, and that means the overall diameter could be from ~21.91"/556.5mm to 24.31"/ 617.5 ((actual width x actual aspect ratio x 2) + wheel diameter) - that is 10.9 % larger/9.9% smaller between extremes (5.45% and 4.95%, respectively, from the nominal diameter) and that makes a BIG difference to gearing!

That seems a lot, and it is taking it to extremes, but I have seen over an inch difference between different makes and brands which were the 'same' size.

Your only real way of sorting it out, for road tyres, is to check out the various web sites of the tyre manufacturers and check the specific figures for the specific tyre types and sizes in question.

As I said, with 'race' tyres they mark what the ACTUAL diameter is on the tyre and it allows changing between tyre sizes, brands, even wheel diameters without changing the actual, effective gearing of the vehicle. It can also allow minor tweaking by fitting a slightly larger, or smaller diameter tyre for small changes.

If you're thinking that seems too easy, you're right, it is - for the same width tyre, the actual tread percentage can also vary a lot - some will have it almost the full width, with 'square' shoulder corners, others will have a narrower tread with 'rounder' transitions at the shoulder.

Then there are all the different compounds, with some using letter, some lower numbers for softer, others higher numbers for softer, and tread patterns - slick, intermediate, full wet, etc.

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