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Tire Math: Calculating the Effects of Tire Diameter on Final Drive Ratio

 

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Thinking about changing the size of your tires?

You’re not alone—people run larger wheels and tires for aesthetic purposes. We’ve also seen guys reduce the overall size of their tires for various reasons. However, there are some serious consequences to any changes to tire diameter.

Tire diameter, along with rear-axle gearing, plays a role in the final drive ratio, which in turn affects acceleration and overall performance. For example, a Camaro that has decent acceleration with a set of 26-inch tall tires and 3.55 rear axle gears becomes a dog with 29-inch tall tires and the same gearing. That’s why it’s important to compare final drive ratios with different tire heights before making any changes to tire diameter. By doing so, you can determine the axle gearing you’ll need to use with your new tire diameter in order to maintain (or enhance) performance. You’ll need three figures to do this: your vehicle’s current rear axle gearing, the diameter of your existing tires, and the diameter of the tires you want to use.

Determining Current Tire Diameter

The easiest way to determine tire diameter is to call your tire dealer and ask for it—easy, but not always an option. The second way is to use the following formulas and figure things out yourself. Please note that these formulas are for metric-rated passenger and light truck tires. Slicks, large off-road, and some larger, Pro Street style tires are already rated by diameter, so you don’t have to do any calculating for them.

We will be using a P235/60R-15 tire as our example: 235 is the section width, 60 the aspect ratio, and 15 the required wheel rim diameter. Here is the basic formula:

Tire Diameter = 2  x  (section width/25.4)  x  (aspect ratio/100)  +  rim diameter

When you plug in the values from our sample tire, the formula looks like this:

Tire Diameter = 2  x  (235/25.4)  x  (60/100)  +  15

Now, let’s do the calculations:

Tire Diameter = 2  x  (9.25  x  .60)  +  15
Tire Diameter = 2  x  5.55  +  15
Tire Diameter = 26.1 inches (round down to 26)

Calculating Effective Drive Ratio

Once you determine the diameter of your old tires and potential replacements, you can compare the effect of new tires on the final drive ratio. For example, if you have 26-inch tall tires with a 3.55 gear, and you want to go to 28-inch tall tires. To calculate the effective drive ratio of the new tires, multiply the diameters together, then multiply that figure by the gear ratio as follows:

26/28  =  .92857142857

.92857142857  x  3.55  =  3.2964 or 3.30

As you can see, the larger tires reduce your final drive ratio to 3.30—a measurable difference. You can also do this to determine the effect of smaller tires on your final drive ratio.

Calculating Equivalent Drive Ratio

Now that you know that 28-inch tall tires reduce your final drive ratio, you can calculate the gearing need to match the performance of your original 26-inch tire/3.55 gear combination. First, divide the new tire diameter by the original tire diameter, then multiply that figure by the original (3.55) gear ratio:

28/26  =  1.0769230769

1.0769230769  x  3.55  =  3.82

With 28-inch tires, you would need a 3.82 rear-axle gear to match vehicle performance with 26-inch tires and 3.55 gears. Since a 3.82 gear is not offered for most rear axles, you will need to choose a gear between 3.73 and 3.90 to get close to the optimum.

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7 Comments

  1. Good article, but what I really want to know is the RPM at 65 MPH and what the speed will be when I am at redline.

  2. 1967 Olds 442?

  3. Pingback: '99 Civic LX manual tranny conversion - Honda Civic Forum

  4. Pingback: Gear ratio question - Ford Mustang Forums : Corral.net Mustang Forum

  5. William Moore says:

    So with my 2.73 ratio and 295-50-15 inch and a 700r trans I’m dead in the water in a drag race lol!

    • Cliff Boyle says:

      Your final drive ratio in overdrive with the convertor locked is 3.00 with those tires. If you have enough torque and can get it to hook you should be just fine in a drag race. I run a 72 buick GS with a built 455 and a final drive ratio of 3.08 and have a best time of 12.53 at 106.02. Not awful for a 4000 lb car. It is all about the combination of parts working together.

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