Axle Ratio/Tire Size Calulator 1.1 - Here are a few calculators that may be useful in deciding tire size, gear ratios, and other stuff. These calculators require Javascript to be enabled and should be compatible with Netscape 3.x and MS Internet Explorer 3.x or above. Special thanks to Mark Medina at for developing these calculators and letting us use them on the site!

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Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed).

Speedometer converter
Original Tire Diameter: 
New Tire Diameter: 
Speedometer Reading (MPH):  Corrected Speed (MPH): 

Now that you have your truck lifted, you want bigger meats. They look cool and have the potential of being very functional, but they may also be robbing you of power/gas mileage. This calculator will get you back to a ratio that's closer to your original. Enter the values on the left and press Compute.

Find new Gear Ratio with new tire size
New Tire Diameter:
Old Tire Diameter: 
Current Gear Ratio:  New Gear Ratio: 

Ever wonder what your low-low-low gear ratio is, your "crawl ratio"? You'll need to do a little research by finding out the transfer case and transmission ratios first. Enter your values on the left side, press Compute and your answer will be on the right.

Find Overall Crawl Ratio
Transmis Ratio:  TransCase Ratio:  Axle Ratio:  Overall Ratio: 

I've spend a bunch of time trying to WAG (wild ass guess) my actual MPH when going down the road. By knowing your gear ratios and tire size you'll get a more accurate idea here. Enter the value on the left, press Compute.

Find MPH
RPM:   Tire Diameter:  Axle Ratio: 
Transmis Ratio:  TransCas Ratio: 

I suppose if you don't have a tachometer you could use this form, otherwise it's only useful to find out how accurate your Tach is. Enter the values on the left, press Compute. 

Find RPM
MPH:  Axle Ratio:  Transmis Ratio: 
TransCas Ratio:  Tire Diameter: 

This formula is a nice way to get the tire diameter of those metric tires that are common on just about everything stock. For example a 265/75R16 would be around 31.6 inches tall. Enter the numbers into this form in the order you read them from the tire and press Compute.

Convert P-Metric tire to Tire Diameter inches
Example: 195/75R15 
Tire Diameter: 
Section Width: 
Section Height: 
Copyright © 1997 by Mark Medina (