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      04-01-2016, 03:08 PM   #1
LUV335
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Tire Size & Speed calculations

I am curious to learn how tire size, if at all, affects speed,distance, traction, stability control etc. calculations.

If I have 18s and go to 20s how does the computer calculate the correct speed? How would the car know that with bigger wheels every revolution covers a little more distance? There is ~11.1% more distance covered per revolution just based on going from a rim size of 18 to 20.
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      04-01-2016, 03:24 PM   #2
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With larger wheels the tire profile is reduced, which means that the overall tire diameter is kept almost the same.

The tire profile is reduced by 5% for every inch in wheel diameter increase.

For example for 225 tires:
225/45-18, tire diameter 25.9"
225/40-19, tire diameter 26.1"
225/35-20, tire diameter 26.2"
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      04-03-2016, 08:31 PM   #3
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Quote:
Originally Posted by .BMW.
With larger wheels the tire profile is reduced, which means that the overall tire diameter is kept almost the same.

The tire profile is reduced by 5% for every inch in wheel diameter increase.

For example for 225 tires:
225/45-18, tire diameter 25.9"
225/40-19, tire diameter 26.1"
225/35-20, tire diameter 26.2"
Thanks. So I guess one has to be careful in choosing the tire and rim size to keep the overall diameter the same? If overall diameter is different it will effect the speed and distance calculations?
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      04-03-2016, 09:36 PM   #4
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Quote:
Originally Posted by LUV335
I am curious to learn how tire size, if at all, affects speed,distance, traction, stability control etc. calculations.

If I have 18s and go to 20s how does the computer calculate the correct speed? How would the car know that with bigger wheels every revolution covers a little more distance? There is ~11.1% more distance covered per revolution just based on going from a rim size of 18 to 20.
Check this out. Much easier than doing your own calculations.

https://www.rimsntires.com/specspro.jsp
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      04-07-2016, 02:07 PM   #5
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Quote:
Originally Posted by LUV335 View Post
Thanks. So I guess one has to be careful in choosing the tire and rim size to keep the overall diameter the same? If overall diameter is different it will effect the speed and distance calculations?
The tire/wheel shop should be able to easily help you find the right combination of wheel and tires so that you maintain as close to stock spec as possible (or do it yourself with that online calculator in the link above).
Anything within 3% of the original overall diameter is considered acceptable and and somewhat within spec.
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      04-08-2016, 08:57 AM   #6
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Quote:
Originally Posted by LUV335 View Post
Thanks. So I guess one has to be careful in choosing the tire and rim size to keep the overall diameter the same? If overall diameter is different it will effect the speed and distance calculations?
Yes, the overall diameter is what is most important. When you make the wheel larger, you will make the tire lower profile. As long you keep the overall diameter close, your vehicle will not notice the difference.

Keep in mind, lower profile tires will ride stiffer and you will have to steer clear of potholes as the lower profile tires do not provide as much protection.
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