Found this article in my local Toyota dealer newsletter.
Great info about tire sizing.
Going Big with Tires and Wheels
Simple tips to get the most out of your new look.
One of the most popular upgrades to make to any car or truck is to install larger wheels and tires. Not only do bigger wheels help update an older car, giving it a more modern look, but they also allow for a wider range of style choices when it comes time to decide on a specific wheel. Larger wheels also offer the opportunity to install wider, lower-profile tires, which can have a positive impact on the handling of a vehicle.
Of course, when looking to install a larger wheel and tire package on your vehicle, there are a number of different factors to consider to help you select an option that gives you both the look and the ride quality that you are looking for. There are a few key concepts you should keep in mind when shopping for a new set of wheels and tires, to make sure that you end up completely happy with your purchase.
One of the most important things to consider is that the overall diameter of your wheels and tires together plays an important role in how your car drives. At the factory, your vehicle’s suspension system was calibrated to function at its peak performance with the stock overall diameter that it rolled off the assembly line with. In addition, the vehicle’s speedometer, and in some cases transmission and ABS systems, rely on calculations that are based on this diameter value. Finally, going with very large wheels can have an impact on the gear ratio in your driveline, potentially slowing down your vehicle’s acceleration.
If you change the overall diameter of your wheels and tires, you might find that your car or truck’s speedometer no longer offers an accurate reading. You also might notice some odd characteristics during cornering, braking or when moving over rough road. At the most extreme, your automobile might feel a bit sluggish off the line compared to how it drove with the original wheels and tires.
Fortunately, there is a very easy system that can be used in order to upsize your wheels and tires and still maintain the original – or very close to the original – overall diameter. Called Plus Sizing, it involves matching your tires to a larger rim in such a way that the original diameter specifications are not significantly altered, if at all. The root of plus sizing can be traced to the sidewall height of a tire. If you look at the sidewall of a typical vehicle, you will most likely notice that there are several inches of rubber between the rim of the wheel and the start of the tire’s tread. This is especially true on vehicles with wheels between the sizes of 15 and 17 inches. Plus sizing involves subtracting an inch or two from this sidewall in order to maintain the proper diameter when using larger wheels. This is possible thanks to the advent of low-profile rubber, which has made it much easier to produce tires that remain strong despite having narrower sidewalls.
To give an example of how someone could move from a 15-inch wheel to an 18-inch wheel without sacrificing the overall diameter of the tire and wheel package, let’s take a look at a few details. A tire with a size of 195/65/R15 – that is to say, a 15-inch wheel with a section width (tread) of 195 millimeters and a sidewall that is 65 percent of that, or 126.75 millimeters – has an overall diameter of 634.49 millimeters once you add the wheel and tire together. If you were to go to an 18-inch wheel, you would have to subtract that extra 3 inches or so from the sidewall of the tire.
Since tires use metric measurements, converting those 3 inches to millimeters would mean a need to reduce the sidewall by a height of around 76.2 millimeters divided by two – since the sidewall adds height at both the top and the bottom of the tire and wheel package. A tire with a size of 205/45/R18 would do this quite nicely, giving us an overall diameter of 641.6 millimeters – well within the recommended three percent safety margin when moving up in tire size.
This simple formula should have you swapping in a new set of tires and wheels that will not only look great, but will also allow you to maintain the same standard of braking, handling and accelerating that you have come to expect from your car.