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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21 Responses to “Understanding Tire Aspect Ratio”

  1. SteveJohnson

    I have a 1969 BOSS 429 Mustang that I have had in storage for 30 years. I recently took it out of storage and am getting ready to get it together and back on the road..
    It currently is sporting Dunlop H50 15″ tires mounted on 15X8 wheels on all four corners of the car….
    A couple of questions?
    1. What is the modern equivalent size to match this
    2. Realizing the multitude of new tire and wheel sizes and dimensions, what other combination would give me the height and width?
    thank you
    Steve Johnson

    • Customer Service

      Hello Steve. The H50-15 tire has a 26.06″ diameter and 11.15″ section width
      The closest modern equivalents are:
      P275/50R15 it has a 25.83″diameter and a 10.8″section width.
      P285/50R15 it has a 26.22″diameter and a 11.22″section width.

      Your car is a rare car, only about 1,500 Boss 429’s were ever built, and
      many would seldom take a quarter million dollar car out on the road much
      less change out the wheels and tires from anything other than the stock.
      But I admire your willingness to take this timeless classic and put it back
      on the street where it belongs. (although it would look a lot better in my
      Wheels and tires can change the look of the car, but are also an expression
      of the driver’s styling taste.

      I like the look of stock sized sidewalls, but many enthusiasts also like a
      little bigger wheel size, opting for 16″ and 17″ wheels.
      Larger wheels are also available but often you’ll sacrifice ride quality in
      doing so.
      As an example one tire size you may consider is:
      P285/40R17 it has a 25.98″diameter and a 11.22″section width.
      Of course you’ll need all new 17″ rims as well.

      Wrench Safe, Mark
      Classic Car Restoration Club

  2. Kevin Madden

    I have a 68 442 what is the beat look and size tires for this vehicle
    I have 14” & 15” wheels Thanks

    • Customer Service

      Great question! The ‘Ask an Expert’ section is currently for members of our online community. By becoming a member, you will have access to our expert’s knowledge. With your membership you will also receive discounts on products and hundreds of hours of Premium content.

      If you are interested in becoming a member, please click on the offer below:
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  3. Keefer

    I have a 2008 MKX with factory 20 inch wheels that are beat up pretty badly from a previous owner. After driving it for over a year now, I have decided I really don’t like the harsh ride of the “low profile” tires it came with. Is it feasible to get a smaller wheel and bigger tire to rectify this issue?

    • Customer Service

      Hello. Yes. The factory had 17″, 18″-and 20″ wheels available for the MKX.
      Lower profile tires do offer less sidewall flex for better handling although the trade-off is a harsher ride and occasional tire sealing issues.
      18″ wheel should dramatically improve your ride quality.
      Mark-Classic Car Restoration Club

  4. Brian head

    Thinking of putting 14″ instead of the 13″ wheels on a 1976 cf bedford van ..back im thinking 14 x12 .and front 14×8 what do you say

  5. Ivan Read

    What is your opinion on the big rim game. 22s and up when your vehicle came with 15s

    • Customer Service

      Hi Ivan. The really Big Rim fad has in many parts of the country begun to fade…
      While there are some advantages in handling with tall rims and low-profile tires there are downsides too…
      Harsher and rougher ride, Rims are very susceptible to damage, ultra-low profile tires prone to bead leaks … these are just a few of the downsides.
      Unless you’re making a statement with your rims the 15 to 20-inch rims seem to be becoming the sizes of choice for most enthusiasts.
      Mark-Classic Car Restoration Club

    • 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.


  6. 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. 🙂

  7. 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.


    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

    • Steve K

      What if you are a low rider and are going for that wider rim look so you can bounce down the boulevard?