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Handy Tire Size Calculator

Posted in car modifications, How To by Kurt Ernst | January 31st, 2010 | 3 Responses |


One of the best performance mods you can make for your vehicle is replacing the stock tires with a good set of aftermarket ones. Why? Because when choosing an OEM tire, manufacturers must balance handling, ride comfort, noise, wear and price. If you guessed that “price” was the single biggest consideration, you’d be right on the money. Upgrading to aftermarket tires allows you to choose the tires that best fit your needs and driving style.

If you’re going for better handling, you may want to purchase a wider tire than stock if it fits on the OEM wheel and doesn’t rub on suspension or bodywork. Changing from a 205/45-17 to a 225/40-17, for example, creates a slightly wider contact patch and should allow for better cornering grip. On the other hand, if you’re purchasing snow tires, a narrower tread will give you better grip on packed snow and ice by applying the vehicle’s weight to a smaller area.

Here’s the problem: the new tires will have a different circumference, which will change the accuracy of your speedometer. How much? This handy GUIDE, courtesy of the folks at Miata.net, will tell you. To use, simply select the stock tire size from the top set of pull down menus. Next, select the new tire size from the bottom pull down menus. The app calculates the difference in tire sizes, tells you speedometer error in percentage and gives an example of speedometer error at 60 mph.

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3 Responses

  1. Joel says:

    Money permitting, I’d love a new set of tires and when the time comes, I’ll come right back here to this post. Thank you, Kurt, you seem very well rounded in this field. Pun intended.

  2. Doug Fachnie says:

    Bloody good idea. My only speeding ticket in 1972 was after I put double oversized snows on my 302 Maverick. 10 over became 17 MPH over limit.

  3. Benito Zarzuela says:

    Good advice, would’ve thought that sticking with the OEM tires of the vehicle maintained optimum performance and up to manufacturer’s specs.

    Look forward to some more insight, regards.