Simple Wheel / Tire Size Calculator
Feb 15Here's something I built the last time I bought new wheels and tires for my Honda Prelude. I knew the stock size of the tires, and I wanted to know what size tires to get if I changed the rim size and width. How big could I go? How wide? How low-profile? And what exactly is an "offset" anyway? 
Tire sizes are composed of three numbers like 185/60/14 or 185/60R14. The first number is the width of the tire in millimeters, the second number indicates the sidewall height, and the third is the rim diameter in inches. Just from this you might think that choosing a new tire size to match the overall diameter of another would be simple. Take the sidewall height, add it to the rim diameter and compare it to the old tire size, right?
It turns out that the second number does not reference the sidewall height exactly, but rather is an "aspect ratio". This means the actual sidewall height can vary depending on the width of the tire specified by the first number, even if the second number doesn't change. This dependency makes calculators for determining wheel and tire size essential and many online tire store websites have them. But of course I wanted to try building my own just to see if I could. 
The default selected Stock Wheel / Tire is the stock tire for a 1992-1996 Honda Prelude. The default selected New Wheel / Tire is the tire and rim size I selected to replace the stock wheels on my Prelude. You can see how the overall diameter of the new wheel is very similar to the stock wheel. This ensures the tire will fit in the wheel well, and also minimize the effect a different sized tire will have on your speedometer and odometer.
As well, while fat tires look pretty sweet, be sure not to select a tire that is too much wider than your stock tires. You may want to physically measure the depth of your wheel wells to make sure the tires won't be rubbing the inner frame or fender. Changing the offset (or adding spacers) may help you fit a larger wheel into the space, depending on its size.
One final note: Don't take my word, or the word of this tool that your selected size of rims and tires will fit on your car. Always double check with your mechanic or other people who own the same vehicle, and even try sourcing that size of tire to try it on before you buy. By using this tool you agree I can't be held responsible for any decisions you may make using the information this tool provides!
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