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 This form can be used to calculate what your new gear ratio will be if you change your tire size. Changing tire dimensions can alter the looks and the performance of your vehicle. Most people do not change their gear ratio when they raise a truck and put bigger tires on it. This will show you why most of them are so slow off the line.

 If you want to know how far off your speedometer will be with your new tires (assuming it was correct with your old ones), enter a number in the "Speedometer Reading" box and click "Calculate" to find out.

Gear Ratio Change with a New Tire Size
New Tire Diameter:
Old Tire Diameter:
Current Gear Ratio: New Gear Ratio:
Speedometer Reading (MPH): Actual Speed (MPH):

 


 Here is a form to calculate the diameter of a metric specification tire. For example :
a 215/45R18 would be approximately 25.62 inches tall. This form can also help you use the form above if you know your tire size already, but don't know what diameter they are.
Converting Metric Tire Measurements to a Tire Diameter
Example: 195/75R14
 /  R 
Tire Diameter:

 


 This form can be used to calculate the theoretical top speed of your vehicle. You will need to know your gear ratio and your tire diameter. If you don't know these, use the two calculators above to figure them out or look in your owner's manual.
Note: This does not account for friction of any type, rev limiters, or MPH limiters, so your vehicle is not actually as fast as this calculator will tell you. It is fun to dream though!
Theoretical Top Speed (MPH)
Gear Ratio:  to 1 
Tire Diameter (inches): 
RPM 
Top Speed:

 


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