Tire Diameters
By Brian Burke

When mounting different tires on your vehicle, it is desirable to maintain the same overall diameter as the original tires. This will help to prevent clearance problems, and will also preserve the accuracy of your speedometer and odometer.

The diameter is calculated from the tire size which is molded onto the sidewall of the tire.

For example: 195/70R14
195 – the width of the tire (in millimeters)
70 – the aspect ratio of the tire (the ratio of the width to the height)
R – radial tire
14 – the diameter of the wheel (in inches)

I don’t know how closely the different tire manufacturers adhere to specifications. Different brands may have slightly different dimensions. Depends on how exact you need to be!

Also please note that these calculations will provide tire diameter for an unmounted tire. When a tire is on a car, the bottom of the tire (the part touching the road) will be flattened a little due to the weight of the car. This will reduce the distance between the hub and the ground. So the “rolling diameter” of a tire is less than the unmounted diameter. Depending on why you want to
calculate the diameter, this may be important to you.

PROBLEM #1:  Calculate the diameter of a tire size 195/70-14 using the following formula:

 diameter = wheel diameter + tire width x aspect ratio
                                                       1270

PROBLEM #2: Lets say your original tires are 195/70-14 as in the example above. So the original tire diameter is 24.75 inches. Maybe you would like a wider tire, like a 235. What aspect ratio should you use so that you maintain the same overall diameter? By moving around the above formula use the following formula:
 

aspect ratio = (desired diameter - wheel diameter) x 1270
                                                tire width

PROBLEM #3 :Calculate the Speedometer Error. Perhaps you want a different tire that will not be the same diameter as the original tires. Maybe you have a friend that will give you a set of 235/70-14 tires. You really want to use them because they are FREE. But what will that do to your speedometer? HINT: You first need to calculate the diameter of the original equipment tire, then calculate the diameter of the optional tire. Use the following formula:

actual speed = speedometer reading x new tire diameter
                                 original tire diameter

PROBLEM #3 : Calculate the Odometer Error. What will the different tire size do to my odometer reading? The new tire diameter is 26.95 and old tire diameter is 24.75 from the above problem. Use the following formula:

odometer error = new tire diameter
                            old tire diameter

So when your odometer increments 1 mile, then you have actually traveled  the you have actually the odometer error!

What happens after 20,000 miles? Use the following formula:

20,000 x odometer error = actual miles traveled