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Gear Ratio, Speedometer, Crawl Ratio, RPM, Tire size Calculators


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 
Old Tire Diameter: 
New Tire Diameter: 
Current Gear Ratio:  New Gear Ratio:

 

 

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

 Speedometer converter 
Original Tire Diameter: 
New Tire Diameter: 
Speedometer Reading (M.P.H.):  Corrected Speed (M.P.H.):

 

 

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 

Transmission Ratio:        

Transfer Case Ratio: 

Axle Ratio: 

Overall Ratio:

 

 

Eliminate that WAG (wild a$$ guess) on your actual M.P.H. 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 M.P.H. 
RPM:    Tire Diameter: Axle Ratio:      
Transmission Ratio:                  TransCas Ratio:

 

M.P.H.: 

 

 

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 

M.P.H.:                

Axle Ratio:                

Transmission Ratio:  

Transfer Case Ratio:        

Tire Diameter:      

RPM: 

 

 

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 Metric tire to Standard 
Example: 195/75R15
Tire Diameter:

 

 

Credit for this calculator goes to Mark Medina. If you would like to see Marks page go here.