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Old 01-16-2013, 04:43 PM
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Reading tire size

I'm hoping this will help other people too. It's one of the things I just can't wrap my head around. How to you read the sizes on a tire? I know the first is the width, second is the aspect ratio, third is wheel diameter. But what does this mean?
Width i get, but what do the numbers mean? Is it mm or something cause the number is always huge.
Aspect ratio seriously tells me nothing. Why not just state the overall diameter of the tire? How does one calculate the diameter?
Wheel diameter is obvious, but what about the width?

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Old 01-16-2013, 04:51 PM
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I agree, it could/should be so much simpler.

The width is in millimeters. It's not the tread width, it's the section width, which is the width of the tire at it's widest point.
The aspect ratio is the sidewall height expressed as a percentage of the width. for example, the sidewall height on a 50 series tire is 50% of the width.

As an example, my daily driver takes a 205/50-16 tire.
The section width is 205 mm. Divide by 25.4 to convert MM to inches = 8.07"
the aspect ratio is 50%. 50% of 8.07" is 4.035"
And, of course 16 is the rim size in inches.

So to figure the tire diameter, you add the rim diameter to twice the sidewall height: 16 + 4.035 + 4.035 = 24.07"

Hope this helps....

Last edited by Joe G; 01-16-2013 at 04:57 PM.
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Old 01-16-2013, 05:00 PM
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or just cheat and go to tire rack.com
and look up your size AND brand/model
and get the section width, the tread width, the height and rim size..
and how many times it'll roll in a mile..

a 215/65/15 tire can have a 1" swing in tread width from brand to brand..
but the section width the same..
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Old 01-16-2013, 06:07 PM
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Here is another helpful website for figuring and comparing wheel and tire sizes:

Tire size calculator

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Old 01-16-2013, 06:58 PM
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Another database: Tire size calculator
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Old 01-17-2013, 09:14 AM
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And dont forget to get the right rating too. It used to be speed rating, now its more of a load rating S,T,H,V,Y,Z, W Staring with S as the lowest ascending to W, You can go up in rating but never down. In other words, put a V rated tire where H is required. All tire size and ratings, along with recommended pressure is inside the drivers door post on any car or truck built after around 1990 or so.
You think the tire business is easy, huh? I have to explain this several times a week, when someone tells me they can get a $39.00 tire for thier BMW!
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Old 01-17-2013, 10:04 AM
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Quote:
Originally Posted by Richiehd View Post
And dont forget to get the right rating too. It used to be speed rating, now its more of a load rating S,T,H,V,Y,Z, W Staring with S as the lowest ascending to W, You can go up in rating but never down. In other words, put a V rated tire where H is required. All tire size and ratings, along with recommended pressure is inside the drivers door post on any car or truck built after around 1990 or so.
You think the tire business is easy, huh? I have to explain this several times a week, when someone tells me they can get a $39.00 tire for thier BMW!
they can.. not everyone drives and needs a 120mph rating.. and they know it..
if it wasn't for lawyers you'd slap that 39.95 tire on it..
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Old 01-17-2013, 12:01 PM
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Quote:
Originally Posted by gearheadslife View Post
they can.. not everyone drives and needs a 120mph rating.. and they know it..
if it wasn't for lawyers you'd slap that 39.95 tire on it..
However, that T rated tire wont stand up to the load a V rated tire will. I explain it this way......You are driving down the road in your Jag XJ8. A lady steps off the curb pushing her baby carriage, you swerve to avoid her......Your 39.00 T rated tire sidewall rolls over and you hit the carriage, however your V rated tire with the stiffer sidewall bites the road, you swerve and every one is happy.
The engineers specify a certain tire for a reason. You put T rated tires on a car requireing V's, not only are you endangering the driver, you are opening yourself for liability.
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Old 01-17-2013, 04:27 PM
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Quote:
Originally Posted by Richiehd View Post
However, that T rated tire wont stand up to the load a V rated tire will. I explain it this way......You are driving down the road in your Jag XJ8. A lady steps off the curb pushing her baby carriage, you swerve to avoid her......Your 39.00 T rated tire sidewall rolls over and you hit the carriage, however your V rated tire with the stiffer sidewall bites the road, you swerve and every one is happy.
The engineers specify a certain tire for a reason. You put T rated tires on a car requireing V's, not only are you endangering the driver, you are opening yourself for liability.
The flaw in your logic is that the speed rating doesn't mean that the tire handles better or has stiffer sidewalls, it only means that the tire is rated for sustained operation at higher speeds. If you've exceeded the 118 mph T rating when you hit that baby carriage, you probably deserve the liability...
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Old 01-17-2013, 05:01 PM
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OK, yes and no. Higher speed rating, higher load index( in most cases) . Here is another number to consider, load index' An explination as per Tire rack
Tire Tech Information - How to Read Speed Rating, Load Index & Service Descriptions
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Old 01-17-2013, 05:26 PM
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Quote:
Originally Posted by joe_padavano View Post
The flaw in your logic is that the speed rating doesn't mean that the tire handles better or has stiffer sidewalls, it only means that the tire is rated for sustained operation at higher speeds. If you've exceeded the 118 mph T rating when you hit that baby carriage, you probably deserve the liability...
The tire ratings are actually something I have a little knowledge on from when i had my truck, and Richie is right, the rating does affect the stiffness of the sidewalls and the amount of weight it will hold. When i got tires on my truck I had to get a higher rated tire because I was using it to tow heavy. I could feel in the ride that they were a stiffer, less forgiving tire over bumps.
So a heavy car on too low-rated tires may cause problems like he described.
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Old 01-17-2013, 05:27 PM
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Quote:
Originally Posted by Joe G View Post
I agree, it could/should be so much simpler.

The width is in millimeters. It's not the tread width, it's the section width, which is the width of the tire at it's widest point.
The aspect ratio is the sidewall height expressed as a percentage of the width. for example, the sidewall height on a 50 series tire is 50% of the width.

As an example, my daily driver takes a 205/50-16 tire.
The section width is 205 mm. Divide by 25.4 to convert MM to inches = 8.07"
the aspect ratio is 50%. 50% of 8.07" is 4.035"
And, of course 16 is the rim size in inches.

So to figure the tire diameter, you add the rim diameter to twice the sidewall height: 16 + 4.035 + 4.035 = 24.07"

Hope this helps....
Thanks! That's a great explanation.
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Old 01-17-2013, 05:29 PM
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The other tire size confusion that I hear from many folks is that they believe a bigger wheel size automatically means a bigger tire diameter. That is because they don't understand that you also have to look at the aspect ratio and section width. For example, I have 245/16" tires on my truck and they are about 31" in diameter because they are 75 aspect ratio. However, the 235/17" tires on my Mustang are only about 27" in diameter because they are a 235 section width and 55 aspect ratio.

Bruce
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