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Discussion Starter #1
Plenty of formulas that use diameter but more precise is roll out. Where can I find something like this?
 

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I agree, but you could measure actual rollout, as in rolling the car on the ground and measuring the rollout, or measuring the circumference with a tape and converting to diameter,
 

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Plenty of formulas that use diameter but more precise is roll out. Where can I find something like this?
What formulas are you looking for? Are you talking about the difference between measured diameter and effective diameter (such as when launching), or just that measuring circumference is more accurate and precise than measuring diameter?
 

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Discussion Starter #6
Circumference is more precise but I was tipped with some basic math (not always my strong point!) on converting circumference to diameter. Then I plug the actual diameter into the online formulas to determine gears, etc. For example, my 99” circumference (14x31 Goodyear 2019) is actually 31.52” diameter (unloaded). 99"/3.14 factor = 31.52”
Thanks to Mike Rietow.
I did find Quarter Pro uses the actual circumference instead of diameter in their calculations if anybody is interested.
 

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It really is basic math: Circumference of a tire (or any any circle) can easily be calculated by measuring the actual diameter of the tire (at a given tire pressure) and multiplying that measurement by Pi (C=D X 3.146146).
I don’t think there’s a single tire manufacturer’s whose tires measures out exactly as advertised, on any dimension. That’s seems to be even worse with slicks, which usually have to be matched in pairs when they're sold and their (loaded) radius changes based on static air pressure and centrifugal force during a run anyhow.
 
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