The last number in a tyre size is the wheel diameter, the size of wheel the tyre is built to fit, measured in inches. In 205/55 R16, the 16 means the tyre fits a 16-inch wheel, and only a 16-inch wheel.
What it measures
The figure is the diameter of the wheel rim where the tyre's bead seats, in inches. It is the one part of a tyre size that has stayed imperial while the width and profile went metric, which is simply a matter of long-standing convention in how wheels are sized. Common car wheel diameters run from 14 inches on small cars up to 20 inches and beyond on large SUVs and performance models.
It has to match exactly
Unlike width or profile, wheel diameter is not a matter of preference or fine-tuning, it is a hard fit. A tyre's bead diameter must match the wheel precisely. A 16-inch tyre physically cannot be mounted on a 15 or 17-inch wheel; it will not seat, and attempting it is dangerous. When buying replacement tyres for the existing wheels, this number must always be identical to what is on the car. Moving to a larger wheel is possible, but it means plus-sizing: fitting a lower-profile tyre so the overall diameter stays about the same.
Changing wheel size
Wheel diameter only changes when the wheels themselves change, for example, moving from steel wheels to larger alloys. When that happens, the tyre size has to change to suit, and the profile is normally reduced at the same time. Going up an inch in wheel diameter while keeping the overall tyre diameter the same means dropping the profile, so a 205/55 R16 might become a 205/45 R17. This keeps the rolling circumference, and therefore the speedometer reading, close to standard.
This trade of wheel size against profile is the core of plus-sizing, and it is what allows a car to wear a bigger, lower-profile wheel-and-tyre package without upsetting how far it travels per wheel revolution.
Matching the wheel to the car
Wheel diameter is one part of fitment; the wheel also has to suit the car's bolt pattern, offset and width, and the tyre has to clear the bodywork and suspension. For a straightforward like-for-like replacement, none of that changes, the tyre simply needs the same wheel diameter as the one being replaced. For anything beyond that, the car maker's recommended sizes are the reference point. For a like-for-like replacement, a search by the current size at a shop that sells tyres online, like Tyres.co.uk, brings up tyres in the same wheel diameter.
From the workshop: this is the number that has to be exactly right. Width and profile have a bit of give in choosing a tyre, but the wheel diameter is fixed by the wheel on the car. Get it wrong and the tyre will not even go on the rim.
Common questions
What does the last number in a tyre size mean?+
It is the wheel diameter in inches, the size of wheel the tyre is built to fit. In 205/55 R16, the 16 means the tyre fits a 16-inch wheel and only a 16-inch wheel.
Why is wheel diameter in inches when width is in millimetres?+
Tyre sizes are a mix of metric and imperial. The width and sidewall are metric (millimetres), while the wheel diameter stayed in inches, matching how wheels have long been sized. It is simply convention.
Can a tyre fit a different wheel size?+
No. The tyre's wheel diameter must match the wheel exactly. A 16-inch tyre cannot be fitted to a 15 or 17-inch wheel. Changing wheel size means changing the tyre size to suit, usually adjusting the profile to keep the overall diameter.
Does a bigger wheel need a different tyre?+
Yes. A larger wheel needs a tyre with the matching wheel diameter, and usually a lower profile so the overall diameter stays close to standard. This keeps the speedometer accurate and the tyre clearing the car.