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Tire sizes...


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So all these tire threads have me wondering about tire sizes...


My wondering starts with our rim width of 8". That's about 203 mm. Someone mentioned (in another thread) about wanting to run 265 mm width tires, which is almost 10.5" of tread width.


Isn't 245 about the limit for 8" rims, or maybe just a bit over? I would think that the closer the tread width is to the wheel width, the better for response. As we go bigger, aren't we adding to the 'ballooning' of the tires, stressing the sidewalls more, increasing flex, and in turn, heat?


I'm a guy - so I abide by the 'bigger is better' rule, but certainly there's an optimal size before other factors (such as above) start impeding the benefits. Let alone the extra inertial mass that comes from more rubber per tire.



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The comment I made reqeusting the 265 was based on trying to best match height and width. This this is all from memory so forgive an errors. The Toyo we are running have a 10.3" width on an 8" rim and a 25.8" height. The 265 Kumho had a 10.4 in width on a 9 inch rim and a height of 25.3". The 245 had a 9.7" width on an 8" rim and a height of 24.3".


Contact area is a function of height and width so the 265 Kumho ended up with a comparable contact area of our current tire.


The other thing that concerns me is that the 245 Kumho is an 1 1/2" shorter than our current tire. I don't know how many of out cars can drop 3/4" and not have problems bottoming on the track. There would also be additional loading and unloading issue.


You pose some interesting questions about mounting 265's on 8" rims. I don't have any answers other than the 45 series aspect ratio of the Kumhos would mitigate some of the sidewall flex concerns.



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Mike - so it WAS you in the other thread. Yup, I just looked again. Anyway...


Interesting about the height differences, but shorter tires = better gearing?


Yeah, it's the sidewall that has me concerned on this. I look at the 35 series on my street car, and pretty much the sidewall is vertical. I look on the CMC car, and the Toyos are ballooning outwards.


If we went even wider, wouldn't that ballooning be more pronounced, and even make the tread want to be more pronounced in the center of the tire too?


Good point about the 45 series on the 265's. But (always at least one, huh?) because they're a shorter sidewall means they'd also have a more direct path to the bead, which would be at least 2 inches narrower on our rims. This is what got the question going in the first place.


Since I have new tires soon down the road, that made me think about the sizes, and not just the Kumho's, but Toyo's too. We may have a 'maximum' size, but that doesn't mean we have to run the maximum, right? I'm just wondering if a smaller width, lower aspect ration tire could provide equal or better performance (in terms of responsiveness, heat dissapation, weight, etc.) than the 'max' that we all have now (because we can).

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Hey Brad,


I'm not so sure a shorter tires equals better gearing. It certainly equals shorter gearing which will be better on some tracks and worse on other.


I did a quick search on tire math and wasn't able to find an easy answer to your questions.


Clearly putting too big of a tire on too narrow of a rim will cause the outside edges of the contact patch to roll up and effectively narrow the contact patch. This effect becomes more pronounced as the aspect ratio decreases. Now to throw some more wrinkles into the mix, tire pressure affects both contact patch width and contact patch length. Lower pressure can compensate to some degree for the previously mentioned roll up.


The rules ceratinly allow for us to run smaller tires and I know Greg Robinson did just that last year. I am willing to bet that Greg won't be going back any time soon. I do think this is a case of bigger is better.



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The 265/45/16 works perfect on the 8" rim,as the A/S guys have been using that size for many years on the 8" rim.

It's 10.4 width is the same on both.

Height is under 6/10ths of an inch.

By the way the 245 is WAY too short.


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