Comment: Tyre Size Calculator
Submitted: Sunday, Jul 13, 2014 at 13:44
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Andrew J8
Why is the measured tyre size (approximately) 5% lower than the online calculator?
I measured the driven tyre of my car, and also a family members car. My tyres is a Good Year, and the other car is using the Yokohama brand. My tyres measured 95.2% lower than the online calculated diametre tyre size, and the other car's tyre was 94.2%. There is appropriate air pressure in tyre, and i checked the wooden ruler with a tape measure to see if there were any innacuracies. There were no anomalies to explain the roughly 5% difference.
Does the tyre really expand that much due to centrifugal force when driving? Somehow i am not convinced the tyre would contort that much. Does anyone know how much the tyre may expand in actual use?
Perhaps it is just the bean counters (ie Accountants from the tyre manufacturers) "robbing" the consumer by giving us less tyre?
I need to know this, because i am calculating gear ratios, top speed, and torque at the wheels for an electric car conversion. Depending on which car and motor combination i use, knowledge of the tyre size could be near critical to my calculations.
Thanks
Reply By: The Bantam - Sunday, Jul 13, 2014 at 22:46
Sunday, Jul 13, 2014 at 22:46
Firtsly you must understand that on line tyre calculators are unreliable.
If you want accurate tyre figures you need to go either to manufacturers specifications or the wheel and tyre standards manual.
If the tyre calculator does not give a different figure for highway and traction treads it is not to be trusted....typically there is a 6mm difference between highway and traction.
Then there are some variations from the generic standard tyre...like traction tyres that have the same overall diameter as a highway tyre..and certain off road tyres that are larger in diameter than the generic standard tyre.
How the tyre diameter and section width are measured is another thing all together.......the tyre is measured off the vehicle after being inflated to the
test pressure at the specified temperature and horisontal.
Don't forget to factor tread wear.
then if you want accuracy, you have to consider rolling circumpherance......that will be different at low speeds than it is at high speeds...as speed increases the tyre tends to stand up.
It realy is worth remembering that tyres are not a particularly accurate thing......but then agian many things about cars are far from narrow tolerance.
cheers
cheers
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Follow Up By: The Bantam - Sunday, Jul 13, 2014 at 22:49
Sunday, Jul 13, 2014 at 22:49
OH another variance.
If you look at a tyre of the same designation but in a different cracase type...like a passenger tyre V a reinforced construction tyre V a light truck V a flotation tyre.....the overall diameter and section width may vary.
cheers
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Follow Up By: Allan B (Member, SunCoast) - Sunday, Jul 13, 2014 at 22:58
Sunday, Jul 13, 2014 at 22:58
Is the circumpherance related to the diahammitor? LOL
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