The middle number in a tyre size is the profile, also called the aspect ratio. It is the single most misread part of a tyre size, because it looks like a measurement but is actually a percentage. In 205/55 R16, the 55 means the sidewall is 55% as tall as the tyre is wide.
Profile is a percentage, not a measurement
A profile of 55 does not mean 55 millimetres. It means the sidewall height is 55% of the section width. To find the actual height in millimetres, multiply the width by the profile:
- 205mm width × 0.55 = roughly 113mm of sidewall
This is why the same profile number describes different sidewall heights on different tyres. A 225/55 tyre has a taller sidewall than a 195/55 tyre, even though both read "55", simply because 55% of a larger width is a larger figure.
What profile does for a car
The sidewall acts as a cushion between the wheel and the road, so its height changes how a car feels.
A higher profile (a bigger number, such as 65 or 70) gives a taller, more flexible sidewall. It rides more comfortably, soaks up potholes and protects the wheel rim from kerb damage. The trade-off is slightly vaguer steering, as the taller sidewall flexes more in corners.
A lower profile (a smaller number, such as 40 or 45) gives a short, stiff sidewall. It sharpens steering response and looks sportier, which is why performance cars and larger alloy wheels use it. The trade-offs are a firmer ride, more vulnerability to pothole and kerb damage, and more expensive wheels to replace if one is bent. For that exposure, many low-profile tyres carry a rim protector, a raised rib that shields the alloy from kerbs.
Profile and wheel size work together
Profile and wheel diameter are linked. Fitting a larger wheel without lowering the profile would make the whole tyre taller, throwing off the rolling circumference and the speedometer. So when a car moves to a bigger alloy, the profile usually drops to keep the overall diameter about the same. This is the principle behind plus-sizing, where wheel size goes up and profile comes down in step.
Choosing the right profile
The profile a car came with is chosen to balance ride, handling and the look of the wheel. Staying close to it keeps the car behaving as intended. Anyone changing it should keep the overall tyre diameter close to standard and confirm the new size still clears the car and meets its load and speed requirements. The recommended size on the door placard is the reference point, and a replacement in the right profile is easy to pull up by size at a well-stocked tyre retailer such as Tyres.co.uk.
From the workshop: low-profile tyres are the ones we replace most after pothole season. A 40 or 35 sidewall has almost nothing between the rim and the road, so one bad pothole can crack an alloy. A taller sidewall takes that hit and shrugs it off.
Common questions
What is tyre profile?+
Tyre profile, or aspect ratio, is the height of the sidewall expressed as a percentage of the tyre's width. It is the middle number of a tyre size, so a 205/55 R16 tyre has a sidewall 55% as tall as it is wide.
Is a lower profile better?+
Lower-profile tyres sharpen handling and look sportier, but they ride more firmly, are more easily damaged by potholes and kerbs, and the wheels they need are dearer to replace. A higher profile rides softer and protects the wheel.
Is the profile number in millimetres?+
No. This is the most common misunderstanding. The profile is a percentage of the width, not a measurement, so the same number gives a taller sidewall on a wider tyre.
How do I work out the sidewall height?+
Multiply the width by the profile percentage. For 205/55 R16, that is 205 multiplied by 0.55, giving a sidewall height of about 113mm.