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These calculators come from Mark Medina's www.4lo.com, and represent some of the common calculations available regarding gears and tire changes.

One of the most common questions I get involves the loss of power involved when you add larger tires. Not only are they often much heavier, but the increased diameter changes the effective gearing of the vehicle, often making it difficult to maintain speed in the higher gears. This can burn up your clutch or overheat the automatic transmission, in addition to requiring more frequent downshifts to maintain speed. Use this calculator to help determine a new gear ratio that brings your engine back into its powerband. 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: 
 

One thing to note about the above results - gear higher than the indicated number to allow for rolling resistance and weight of the tire. My personal change from stock 27" tires to 32" with 3.55 gears indicates I should use 4.20 gearing. While there's no 4.20, there's 4.10 and 4.56 available. 4.10 seems closer, and it is but it wouldnt have helped with the downshifting or the added weight of the tires. I'm now geared low enough to crawl off-road yet still get up and go on the highway.

Insert Speedometer Check

Insert Metric Tire Size Conversion

Insert Speedometer Check

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created: Jan 15, 2005