Tire aspect ratio

Most people are perfectly aware of the importance of the tread, but there is a lot to be said about tire profile too. Height of the sidewall influences the load index, maneuverability and driving comfort.

Tire sidewall is where you can check the tire manufacturer, model, size, speed and load indexes, UTQG rating, load and pressure marking, DOT code and additional parameters, such as the snowflake symbol for mud+snow tires or original equipment designation. The sidewall itself, however, can already tell you a lot about the tire’s performance. Its height, or aspect ratio, influences the looks and the performance of the whole vehicle.

Low profile tires fit sporty cars

Low profile associates with stylish, sporty cars used for racing

Calculating

The most common tire size designation method is the P-metric size, so for example, P225/50R16. The aspect ratio here is 50. It means that the sidewall height is 50%, so half of the tread width, which in this case is 225 mm (8.86”). The sidewall is therefore 112.5 mm high (approx. 4.40”). As for the remaining values, we have the rim diameter (16”), the P stands for “passenger car tire” and the “R” means the tire has a radial construction. How to calculate the tire aspect ratio?
 

  P245/35R20 P195/65R15 P225/60R16
Tread width
(millimeters)
245 mm 195 mm 225 mm
Aspect ratio
(% of tread width)
35% = 85.75 mm (±3.38”) 65% = 195 mm (±4.99”) 60% = 126.75 mm (±5.35”)
Rim diameter
(inches)
20” 15” 16”

Interpretation

The height of a tire influences several other important parameters. More rubber and internal cords means that the tire with a much higher aspect ratio is heavier. The weight has to be multiplied by four, for a full set of tires in a car, so the difference can be quite noticeable. What you gain in return is increased volume of pressurized air inside the tire, and with it – much higher load capacity. High tire sidewalls are more flexible, which results in a more comfortable ride.

Prestigious racing cars use low profile tires, because they drive on level terrain and they need good cornering. Stiff sidewalls provide a better reaction time. While such tires are more of a problem than improvement in a regular passenger car driving on typical highways and roads, low profile tires give cars sporty looks. The aspect ratio started to decrease with each year. In the past, most cars had tires with a very high sidewall – usually narrow, bias-ply 85-series to 60-series tires with a characteristic white wall. The tendency led to a decrease in aspect ratio of original equipment tires from the initial 80% or 85% to the average of 60%, 55% and 50% that we commonly see today.

Nexen low profile tire

Ultra-low profile Nexen tire, 365/15R24

Lowering the aspect ratio even further will appeal to many, but will also have a serious impact on comfort. You can compensate for the reduced air volume (responsible for load capacity per tire) by mounting wider tires, but the ride will remain uncomfortable on uneven terrain, as a stiff sidewall is less flexible. For the looks, though, people can do anything. To go extreme – you can see products such as the Pirelli Scorpion Zero 305/30R26, Toyo Proxes 4 275/25R28 or a more economical 30 inch low profile Wanli S-1089 275/25R30 tires. These look more like rubber bands attached to rims and are mounted on hi-risers for their hilariously exaggerated looks. For showing off, they may just be perfect - for driving, not so much. For improved comfort, reasonable load capacity, good traction and cornering, is seems best if the aspect ratio remained between 50% and 60%  that constitute the standard.

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