Dear Wheel Doctor,

I have a car with tires on it. The tires need replacing. What is the best tire for my car?

Thanks,

A. Reader

Okay, so I rarely get letters that are quite this obtuse, but in some ways they may as well be, because there are so many factors in selecting a tire that it could make even an experienced tire professional’s head spin. To aid in the selection process, here are the factors that go through my head when recommending a tire.

Considering these factors can be overwhelming, so here is a breakdown of what to consider before or even while consulting with a tire sales professional so you can express what your needs and priorities are.

Tire Size:

When you bought your car, it came with a set of tires in the size that the carmaker decided was correct. This is what we call the OEM (Original Equipment Manufacturer) tire size. For most drivers, there will never be a need to deviate from this size, and quite a few reasons not to do so. Your tire size can be found embossed on the sidewall as a set of numbers separated by slashes, as in 195/50/16. The first number (195 in this example) is the width of the tire in millimeters. The second number is the ratio between the height of the tire and it's width (50% of 195mm) The third number is the inside diameter of the tire in inches, which corresponds to the outside diameter of the wheel (16”).

The thing about tire size is that there are many settings on your car that depend on the tire size being correct.

The speedometer and odometer settings are both predicated on the distance it takes for your tire to turn one revolution, otherwise known as the outside diameter of the tire.

If you are simply replacing your tires, you will want the same size. Trust me, you don't want to get into the issues related to changing tire sizes unless you absolutely have to, and there are really only two reasons to do so. The first reason is if you are changing the size of your wheels. If, for example, you are going up a size from 16” to 17” wheels, you will obviously need to also have 17” tires. The issue here is that as you gain an inch in wheel diameter, you have to lose an inch in tire height to keep the outside diameter of the tire the same as it was. This is called Plus One Sizing, and while the process can be headache-inducing, there are luckily quite a few tire size calculators online to help you get it right.

The second reason is if you have an odd tire size that's not easily available and need a different size within the same diameter. In that case it is possible to change the width of the tire, but this must be done very carefully, paying attention to two factors – that the ratio must also change to ensure that the outer diameter remains the same and that shape of the tire's air chamber remains at least roughly comparable, and that the width of the tire remains compatible with the width of the wheel.

This last part is very important and often overlooked. Far too often I've seen people who changed to tires that were simply too wide or too narrow for the wheels they were on, putting the tire sidewalls at an angle in order to meet the wheel flanges. That's an incredibly dangerous proposition, because the sidewalls are no longer able to do their job if they're at an angle like that. Any respectable tire shop will have a book that shows which tire widths are approved for a given tire size, so if you have to change the width, make absolutely sure that your installer checks that out.

Tire Ratings:

You will also want to consider how your tires are rated. There are three ratings to consider; load ratings, speed ratings and UTQG ratings.

Load ratings are shown on the tire sidewall as a two-digit number that corresponds to the amount of weight the tire is expected to be able to carry. For example, a load index of 96 means the tire can carry 1,565 pounds, for a total of 6,260 pounds on all four tires. A tire with a load index of 100 could carry 1,764 pounds. In general, you do not want to install a tire with less of a load rating than the OEM tires had.

Speed ratings essentially describe what range of speed to tire is built to endure for a long period of time without coming apart. It is shown on the tire sidewall as a letter code next to the load rating. In general a speed rating of H or above is sufficient for most summer applications, while snow tires are often T rated. However, it has become a trend for OEM tires to be rated V or above, even if it's not strictly necessary, and many installers will not put on tires rated lower than the OEM tires for liability reasons. It's also the case that higher speed ratings generally indicate that the tires are built tougher, often with extra steel belts or nylon cap plies, and this will help them perform better.

Uniform Tire Quality Grading (UTQG) ratings are supposed to give a uniform sense of the tire's traction, heat dissipation and treadwear capabilities. In practice they are perhaps less empirical and/or useful than they should be, but they do provide some ability to compare different tires in a standard way.

Then, beyond the purely technical aspects, there are the more subjective factors for choosing the right tire.

All Seasons or Snow Tires:

If you live in an area where you get any kind of serious winter, you'll be needing to decide whether to put on winter tires when the snow starts to fly. That basic decision will then determine whether you go with pure summer tires or whether you get a set of all-season tires that can stay on all year long. This decision is made somewhat more complex by the fact that many tires that are labeled as all-season are not really all that good in winter conditions. Here are some articles that can help with that:

All Seasons or Snow Tires?

The Best Performance All-Season Tires For 2013

The Best Winter-Biased All-Season Tires For 2013

What You Need to Know About Winter Tires

The Top 10 Dedicated Snow Tires for 2013

Ride Quality and Performance:

Only at the end of this long chain of information and decisions should you begin to look at what kind of ride and performance you want out of your new tires. This depends largely on how you drive – do you value a nice smooth ride over racecar grip? You want Grand Touring tires. Do you need an inexpensive set so that your teenager can master parallel parking without breaking the bank? You want a good set of daily drivers. Do you want your tires to have low rolling resistance so you can save on gas? You want something like the Bridgestone Ecopia or Yokohama Avid Ascends. Want to push your sportscar to its limits. You'll want to look at UHP tires. These are highly personal decisions, but they are important if you want to be happy with the tires you choose.