Hobbies Cars & Motorcycles Tire Size Calculators You Won't Need One Until You Need It Share PINTEREST Email Print Westend61 / Getty Images Cars & Motorcycles Cars Tires & Wheels Buying & Selling Basics How Tos Reviews Tools & Products Classic Cars Exotic Cars Corvettes Mustangs Motorcycles Used Cars SUVs Trucks ATVs & Off Road Public Transportation By Sean Phillips Updated February 16, 2019 Here's to one of those things you never need until you really need them: tire size calculators. You never need one of these until you've decided to change to a different size of tire, at which point they become essential because even Stephen Hawking doesn't want to do that kind of math in his head. A car's speedometer and odometer settings are determined by the overall diameter of the wheel and tire assembly, or the circumference of the tire, essentially the same thing. So when you change the wheel diameter by an inch, say by going from 16” to 17” wheels, your tire must have an inch less sidewall height to keep the same overall diameter. If you don't put the right “plus-one” sized tire on, your speedometer will be giving you incorrect readings. In general, you want the tires to have less than 1% difference. Ideally, you want less than 0.5% difference. So whether you're upsizing or downsizing your setup, you're probably going to want a tire size calculator. Below are the best we've found. Which one is right for you depends on your specific needs. Miata.net The tire calculator at Miata.net gives only the information really needed to decide on the right size for a customer – the difference in overall diameter expressed as a percentage and the difference between the speedometer and the actual speed at 60mph. A number substantially less than 1% - 0.1% is optimal because “This size would mean that when your speedometer says 60, you're actually doing 59.9, and that's about as good as it gets.” This is a very easy to use basic app with a nice graphic component that makes it easy to visualize two tire sizes together. 1010 Tires This calculator is second choice. It allows for more than two sizes at once and gives some more in-depth information. Tacoma World Here's one with a few more bells and whistles. TacomaWorld's calculator offers all the usual tire size data in inches, millimeters, and percentage of difference. It seems to use the same little tire graphic as the calculator at Miata.net for a clear visual of the two sizes being compared. It also gives the speedometer discrepancy in 5-mile increments from 20mph to 65mph, and even information on RPM's and gear ratios. Gear ratios! A great calculator for the serious techhead. Discount Tire Discount Tire's relatively simple calculator gives dimensions in inches without percentages. It does allow you to input a speedometer value to get the difference at that speed. It also shows some nice graphics at the bottom so that you know what they're talking about. Kouki Tech Kouki Tech has a decent calculator on its site as well. The graphic aspect is a bit sexier than most, which actually makes it somewhat tougher to get a clear idea of the sizes. It's an easy and straightforward kind of calculator, with a lot of good information on the bottom of the page, but it needs to give out some more data to really compete. Version 2 of Kouki's calculator is apparently "under development," although it appears to have been under development since sometime in 2009. WheelSizeCalculator.com This one is quite interesting. Not a tire size calculator, per se, this calculator will plug in your car's make and model and give you the proper wheel sizes, including data for Bolt Circle Diameter and Offset, making this calculator incredibly useful for people who work with wheels. It also gives every possible tire size that will fit on the wheels, as well as the proper wheel width for the tire. It's kind of tough to read and understand, but it's a powerful use of the data, and it gave me an idea – every tire size calculator should really start including the proper wheel width for the tire size in question. Now that would be useful.