Need Help w/ Understanding Tire Sizes - Toyota Tacoma Forum
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#1 (permalink) Old 02-25-2004, 01:49 PM
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Need Help w/ Understanding Tire Sizes

I know the last # means the size of the rim, but what do the first two numbers mean?

For example, what is the difference between a 265/70R16 vs. a 265/75R16?
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#2 (permalink) Old 02-25-2004, 01:54 PM
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Re: Need Help w/ Understanding Tire Sizes

the second number is the width, and the first number is the circumfrence, or height, something like that. a 265 is a 31" tire, and a 275 is a 32", and a 285 is a 33".
#3 (permalink) Old 02-25-2004, 02:11 PM
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Re: Need Help w/ Understanding Tire Sizes

The first number is the section width in mm (not the tread width) the section width is the widest part from sidewall to sidewall a tire with a section width of 10.5 inches (265mm) will be anywhere from 9 to 9.5 inches at most. The second number is a ratio of the section width to sidewall height. In other words if you have a 265/75/16 the section width is 265mm or approximately 10.5 inches the sidewall height is 75% of that width and 16 is the rim diameter. Any more questions let me know and I will try to clarify more.
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#4 (permalink) Old 02-25-2004, 02:18 PM
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Re: Need Help w/ Understanding Tire Sizes

actually, the first # is the distance in mm from the bead of the tire to the center and the second # is the percentage of that first # that is the side wall.

for example 265/70/16 (70% of 265 is 185.5) wich means the side wall is 185.5 mm tall and the 79.5mm that is left from 265 (265-185.5=79.5) is 1/2 of the contact patch/surface/width so total contact area would be 159mm (79.5x2=159)
(it ends up being a 30.6 inch tire)

my 215/35/18's have a side wall of 75.3mm and are 279.4mm wide and 23.9 inches tall
hope this shows you how it works
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#5 (permalink) Old 02-25-2004, 02:23 PM
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Re: Need Help w/ Understanding Tire Sizes

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my 215/35/18's have a side wall of 75.3mm and are 279.4mm wide and 23.9 inches tall

This would mean that your tire is over 10 inches wide (279.4mm/25.4) are you sure thats right?
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#6 (permalink) Old 02-25-2004, 02:29 PM
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Re: Need Help w/ Understanding Tire Sizes

This link was posted in the 2wd tire and wheel section.
http://www.tirerack.com/tires/tiretech/general/size.htm
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#7 (permalink) Old 02-25-2004, 02:32 PM
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Re: Need Help w/ Understanding Tire Sizes

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The first number is the section width in mm (not the tread width) the section width is the widest part from sidewall to sidewall a tire with a section width of 10.5 inches (265mm) will be anywhere from 9 to 9.5 inches at most. The second number is a ratio of the section width to sidewall height. In other words if you have a 265/75/16 the section width is 265mm or approximately 10.5 inches the sidewall height is 75% of that width and 16 is the rim diameter. Any more questions let me know and I will try to clarify more.
This is correct.

To convert to inches, use this formula:
((Width * Aspect Ratio * 2) / 2550) + Wheel diameter = height in inches

For example, my 215/75R15 tires would be as follows:
((215 * 75 * 2) / 2550) + 15 = 27.64" tall
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#8 (permalink) Old 02-25-2004, 02:34 PM
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Re: Need Help w/ Understanding Tire Sizes

Actually the first number is you width in millimeters, divide by 25.4 for inches. The second number is you aspect ratio or percent factor of your side wall height to your tire width. The 3rd number is your wheel diameter. For a 265/70/16 tire your width is 10.43 inches(265/25.4) and your side wall height is 7.3 inches(10.43x70%). Your total diameter is 30.6 inches or twice your side wall height plus wheel diameter.

Looks like I was a little late in posting...gotta hit that send button faster.
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#9 (permalink) Old 02-25-2004, 02:39 PM
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Re: Need Help w/ Understanding Tire Sizes

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Actually the first number is you width in millimeters, divide by 25.4 for inches. The second number is you aspect ratio or percent factor of your side wall height to your tire width. The 3rd number is your wheel diameter. For a 265/70/16 tire your width is 10.43 inches(265/25.4) and your side wall height is 7.3 inches(10.43x70%). Your total diameter is 30.6 inches or twice your side wall height plus wheel diameter.

Looks like I was a little late in posting...gotta hit that send button faster.
I thought that I had heard that somewhere before
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#10 (permalink) Old 02-25-2004, 02:39 PM
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Re: Need Help w/ Understanding Tire Sizes

hummm. i don't know where i went wrong, the math seems right but i just went and measured my tires and they're a little over 8"s wide wich is about 209mm wide, but thats not what the math says that weird
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