When it comes to selecting a new set of tires for your daily driver or weekend cruiser you have a few decisions to make and a ton of information sources. Picking the right tire is one of the most overlooked areas on many cars. Keep in mind that ultimately it is the tires that deliver power to the ground and the only contact point with the road, so choosing the right tire size is essential for maximum performance and to ensure adequate safety.

Many online sources for information on tires are either trying to convince you to buy their tires, or include too many overwhelming details for off-road vehicles and trailers. This guide will tell you exactly what you need to know to pick the right tire size for your street cruiser or daily driver.

General Tire Size Information

Tire diameter is determined by three basic numbers located on the outer sidewall following a format similar to 225/45/17. These numbers represent the width, sidewall profile/height, and rim diameter. I will attempt to explain their meaning in the most common street-driven applications. This would be a good time to get used to dealing with millimeters (mm) and percentages.

This three-digit numeric portion represents the tire's width in millimeters. Using our 225/45/17 example we can see this tire is 225 millimeters at the widest points from sidewall to sidewall. Everything else being equal, a wider tire will generally offer better dry traction. However, it is important to consider a few things before just buying the widest tire available. Wider tires tend to be heavier and require more energy to compensate for the increase in inertia caused by the rotating mass now being further from the center of the tire. Wider tires also require more room in the wheel well and may require suspension modifications to clear other components in the fenders.

It is also important to understand that a tire's width is directly influenced by the width of the wheel it is mounted on. A tire that is mounted on a narrow rim would give the appearance of a bulge on the sidewalls and offer a more narrow contact area. Stickier tires can offer superior performance without increasing the width, so remember this if you're unable to mount wider tires on a set of wheels you are obligated to keep.

It's generally accepted that wider tires offer greater performance. However, you must keep a balance so the disadvantages never outweigh the advantages. I encourage everyone to try new things with their rides, but tires is one of those few instances where it pays off to go with what has worked for others in the past.

The second two-digit number in the sequence identifies the tire's profile or aspect ratio. This measurement can be referred to as profile, aspect ratio, or sidewall height. A tire's profile is the distance from the wheel to the tread and it's represented by a percentage of the tire's width. A 225/45/17 tire's profile can be expressed as 45% of the 225mm width. 225mm x .45mm gives us a sidewall height of 101.25mm or approximately 101.25mm / 25.4 = 3.99 inches. The higher the number the taller the profile and vice versa, so a tire like 225/30/17 would give us an extremely thin sidewall of 67.5mm or 2.66 inches.

Understanding sidewall profile is just as important as choosing the right tire width. Not all tires are built the same way so generalizations like "bigger profiles gives the best drag traction" and "low profiles are the best handling" should be taking with a big grain of salt. A tire's material will have a bigger impact on traction and turns than just a sidewall number. Keep in mind that a higher profile tire results in a bigger tire diameter which can rub on the fenders or suspension component, while a low profile can sacrifice ride comfort and expose the wheel to more hazards on the road.

The last number in our example 225/45/17 indicates the wheel size required for mounting the tire. This is the distance from bead seat to bead seat across diameter of wheel and it must be the same as tire's inner diameter. Basically, the outer diameter of the wheel must match exactly with the inner diameter of the tire. Mounting a tire on a wheel of diameter diameter can result in serious problems and even death.

Overall Tire Diameter

Let's take a minute to discuss overall tire diameter, the distance from one side of the tire to the other through center of the rim. We can use what we already know from the three previous numbers to calculate a tire's diameter. We can now combine our previous calculations to determine diameter. We know that a 225/45/17 tire has a width of 225mm with a sidewall profile of 45% resulting in: 225 x .45 = 101.25mm or 3.99 inches. The overall diameter is then the sidewall multiplied by 2 plus 17 inches for the wheel's diameter, so 17 + (3.99 x 2) = 24.98 inches. Now that you've learned the math behind it you can use this to easily find the tire you need.