Whats The Difference In These Tire Sizes?

sab316 · February 25, 2010

I tried the search function but this is a hard one to search. Whats the difference in say a 285/70/17 and 33x12.50R-17? I know how the 285/70/17 numbers work and its a 33" tire. I know the second one is a 33" tire but whats the 12.50? Why do they decide to list it like that?

4x4txed · February 25, 2010

Stock Tire - 285/70R17

Section Width: 11.22 in 285 mm

Rim Diameter: 17 in 431.8 mm

Rim Width Range: 7.5 - 9 in

Overall Diameter: 32.70 in 830.58 mm

Sidewall Height: 7.85 in 199.39 mm

Radius: 16.35 in 415.29 mm

Circumference: 102.7 in 2608.5 mm

Revs per Mile: 636.0

Actual Speed: 60 mph 100 km/h

Tire 1 - 318/66R17 (33x12.50R-17)

Section Width: 12.51 in 318 mm

Rim Diameter: 17 in 431.8 mm

Rim Width Range: Unrecognized tire size

Overall Diameter: 33.52 in 851.40 mm

Sidewall Height: 8.26 in 209.80 mm

Radius: 16.76 in 425.70 mm

Circumference: 105.3 in 2674.6 mm

Revs per Mile: 620.5

Speedometer1: 58.5 mph 97.5 km/h

Speedometer Difference: 2.497% too slow

Diameter Difference: 2.45%

http://www.1010tires.com/tiresizecalculator.asp

Tire sizes for vehicles, especially larger tires for trucks and suv’s, can be described using several different size standards. The most commonly used standards for regular light-duty trucks and SUV’s are:

* Standard Inch Size- Ex: 31×10.50r15

* Metric Size- Ex: 265×75r16

Standard Inch Size

The breakdown of a tire’s size in standard inch measurements is straightforward. We will look at a common tire size for many stock SUV’s and Trucks:

*Size: 31×10.50r15

The size of a tire is split into three components. First comes the overall diameter (in this case, 31 inches), followed by the tire’s tread width (10.5 inches) and finally the diameter of rim it is made for (15 inches).

Metric Size

Metric size tires are somewhat more confusing, because the tire’s size is actually a mixture of metric and inch units. We will look at a common tire size:

*Size: 265x75r16

In the “modified metric system” or “U.S. metric system” as it might be called, the width of the tire’s tread is called out first, in this case 265 mm (milli-meters). The second number describes the height of the tire’s sidewall, but it is described as a percentage of the width of the tire. So, in our case, the tire’s sidewall is 75% the width of the tire, or 199 mm. Finally, the third number is the diameter rim the tire is designed for, but because the U.S. uses inch-size rims the diameter is described in inches (in other parts of the world, this number may instead be described in millimeters). In our case, the tire is made for a 16 inch rim.

Converting metric to standard inch size

Given a metric size, it takes a few operations to convert to inch size. We will convert the following metric size to inches:

* Metric Size: 265x75r16

The most important thing to remember is that there are 25.4 mm (millimeters) in an inch. Finding the tire’s with is a simple calculation, you simply divide the tire’s width in millimeters (in our case 265 mm) by the number of millimeters in an inch (25.4 millimeters per inch):

*Tire Width: 265 mm / 25.4 mm per inch = 10.43 inches

Finding the tire’s overall height is more difficult. We have already found the tire’s width in inches, now we need to find the tire’s sidewall height by taking the tire’s width (10.43 inches) and multiply it by the percentage wuoted in the tire’s size (in our case 75% or 0.75):

* Sidewall Height: 10.43 inches * 0.75 = 7.82 inches

now that we have calculated the sidewall height, we can find the overall diameter of the tire by adding two sidewall heights (7.82 inches) and the rim diameter (16 inches) together:

* Tire Diameter: 7.82 inches + 7.82 inches + 16 inches = 31.64 inches

So, based on our calculations, a 265x75r16 tire is approximately equal in size to a 31×10.50r16 tire.

Converting Standard Inch to Metric Size

It can be useful to know how to convert from inch size to metric, in order to know what a certain size tire would correspond to in the metric sizing standard. We will back-convert the following tire size from the above metric to inch conversion:

*Inch Size: 31×10.50r16

First, we already know the width of the tire (10.5 inches) so we can convert that number to millimeters by multiplying by the number of millimeters in an inch (25.4 millimeters per inch):

* Tire Width: 10.5 inches X 25.4 mm per inch = 266.7 mm

Metric sizes are most often quoted in even divisions of 5 mm, so we can round the above result down to 265 mm.

Next, we need to detemine the sidewall height. To find the sidewall height, we take the tire’s overall diameter (31 inches), subtract the diameter of the rim (16 inches), and divide that number by 2:

*Sidewall Height: ( 31 inches - 16 inches ) / 2 = 7.5 inches

Now that we know the sidewall height, we can find the percentage value used in the metric sizing standard by dividing the sidewall height (7.5 inches) by the tread width (10.5 inches):

*Sidewall Percentage: 7.5 inches / 10.5 inches = 0.71 = 71%

Like the tire width, the tire’s sidewall percentage is quotes in units of 5, so our tire sidewall percentage is somewhere between 70% and 75%.

So, based on our calculations, a 31×10.50r16 tire is approximately equal in size to a 265x75r16 tire.