Understanding Tire Aspect Ratio

Tire Aspect Ratio One aspect of tire sizing that is often confused is aspect ratio. The aspect ratio is a tire’s section height as compared to its section width. The aspect ratio is often referred to as the tire’s “series.” With that in mind a 50-series tire has a section height that is 50-percent of the section width.

Of course, the section width can change on a tire in relation to the width of the wheel on which it’s mounted. As the aspect ratio decreases, it is important to use a wheel that is closer to the section width. For example, a 50-series tire and larger should have a wheel that measures 70-percent of its section width, and a 45-series tire should have a wheel that measures 85-percent of its section width.

The ratio between the section width and the wheel width is very important in selecting the right wheels and tires for your ride. Choosing a wheel that is too narrow will cause the sidewall to balloon and will allow the tire to distort excessively under hard cornering, resulting in sloppy handling. Choosing a wheel that is too wide for the tire can cause it to pull away from the wheel at high speed and will give the car a much harsher ride. The loss of curvature in the sidewall will not allow the tire to flex as far, resulting in a ride where every pothole feels like a crater.

Most tire manufacturers publish guides for their various tire lines, complete with the range of wheel widths that are best suited for each tire size. Sometimes the best tire for your car is not necessarily the biggest tire that will fit, and taking the time to thoroughly explore your options before you pull out your wallet will reap far greater rewards down the road.

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9 Responses to “Understanding Tire Aspect Ratio”

    Interesting article.
    Could you address the effect of wheel size on ride and handling. For example, what is the effect of, say, going from a 15″ wheel to 16″ or more.
    Thank you.

    • Customer Service Techs

      Tall wheels are popular among many enthusiasts and can give the car a distinctive look. Although for focus in answering your question, we’ll assume the “overall diameter” of the tire remains the same and the wheels get larger while the tire sidewall gets smaller.
      With that said, larger wheels and shorter sidewall tires, first became popular with performance enthusiasts, the short sidewalls flexed less giving the car more solid feel when turning and braking hard. Of course there is a trade-off, low-profile tires loose their ability to absorb shocks from potholes, road debris or simply bumps in the road, making for a much rougher ride.
      Using your example, most enthusiasts will not notice much of a difference between 15” and 16” wheels, and from my own experience you begin to really feel the road at wheel sizes 18” and above.
      It’s worthy of note, If a vehicle is going to be driven on the roads, the narrower the tire sidewall the more prone it will be to sustain wheel damage or breaking the bead on the tire when encountering potholes and alike.
      Wrench Safe, Mark

  2. lapswimmer916

    Assume you went from a 26″ tire OD to a 25″ OD, but kept the same wheel. How much would that change effectively increase your final drive ratio. Let’s say it’s got a 3.45:1 rear end with a 1:1 final drive gearing.

  3. murryfrankc

    My 76 Grand Prix had G70-15 tires, on 15×7 stock rims. I think “G” is appox. equal to “215” metric size. So i did some calculations for wider tire and rim sizes that would be close to the stock tires diameter. I went with 245/60/15 on 15×8 in front and 295/50/15 on 15×10 in rear. 4″ backspace on front and 5″ on rear. No clearance problems…almost exactly the same diameter. Very good improvement in handling and decent ride quality. And it looks Bad Ass. Installed Monroe Air Shocks to adjust the overall stance and new cargo coils in the rear.Happy with this set up. 🙂

    • Customer Service

      Dear Cal,

      Thank you for your patience while I get a response from the experts:

      There is nearly 2″ difference in the two tire sizes you listed.
      I would consult a tire shop to determine if there would be any interference issues and know that your Speedometer and odometer readings would not be correct.

      Wrench Safe,

      Classic Car Restoration

      • cal

        Thanks for the reply There seems to be be plenty of room in the wheel arch and I know it would affect speedo.