Tire size calculator
The size is not the only thing moulded into that sidewall. Find the DOT marking and the last four digits are the week and year the tyre was built: 3419 means the thirty-fourth week of 2019. Rubber ages whether or not it is driven on, and a tyre with full tread and a decade behind it has hardened enough to matter in the wet. Tyre manufacturers commonly advise replacement at ten years from that date regardless of wear, and sooner on anything that spends most of its life parked, which is why a caravan or a rarely used second car tends to need tyres long before it needs tread.
On a 205/55 R16 the sidewall is 55 per cent of 205 mm, so 112.75 mm. Overall diameter is the 16 inch rim, 406.4 mm, plus two sidewalls: 631.9 mm, or 24.88 inches. A replacement is normally kept within about three per cent of that, which is the convention that keeps the speedometer inside its type-approval band.
How to read and compare tyre sizes
The code mixes three unit systems and none of them announce themselves. The 205 is the section width in millimetres, the 55 is a percentage rather than a measurement, and the 16 is inches. That is a historical accident nobody has tidied up, and it is why the overall diameter has to be calculated rather than read. Two more things sit on the same sidewall that this does not take: the load index and the speed symbol, the 91V on the end of the code. Fitting a tyre below the load index the vehicle was approved with is a real problem, and no amount of matching the diameter fixes it.
Section width is also a slipperier number than it looks. It is the widest point of the inflated, unloaded tyre measured on a specified rim, and it includes the sidewall bulge and any lettering moulded into it. The tread that actually touches the road is narrower, sometimes by a couple of centimetres. The same tyre on a wider rim measures wider still, which is why manufacturers publish a measuring rim alongside the size. Treat 205 as an identifier rather than as the width of your contact patch.
Plus-sizing works because two changes cancel. Going from 205/55 R16 to 225/45 R17 gives 634.3 mm against 631.9, a difference of 2.4 mm or 0.38 per cent, so the speedometer, the ABS and the stability system all still see what they were calibrated for. Get it wrong and everything downstream of wheel speed drifts together: speedometer, odometer, cruise control, trip computer fuel economy, and in many cars the traction and stability thresholds. Stay with the same 16 inch rim and go from 205/55 to 215/55 and the diameter jumps 11 mm, 1.74 per cent, because widening a tyre at a fixed aspect ratio makes the sidewall taller as well.
One caveat about the circumference shown here. It is the geometric circumference, π times the diameter, of a tyre standing free. A loaded tyre deflects, so the distance it actually covers in one revolution is two to three per cent less than that, and the revolutions per kilometre are correspondingly a little higher than the figure above. For comparing two sizes against each other the geometric number is exactly right, since both are computed the same way. For calibrating something against real distance, use a manufacturer’s published rolling circumference.
What people use it for
- Checking an alternative tyre size before ordering
- Plus-sizing to a larger wheel without changing the diameter
- Explaining why the speedometer reads higher than the satnav
- Working out revolutions per mile for a gearing or rev calculation
- Comparing a winter size against the summer fitment
- Checking what a worn tyre does to indicated speed
- Decoding a sidewall on a used car before buying
- Seeing how much a wider tyre raises the ride height
Questions
205 mm section width, a sidewall 55 per cent of that at 112.75 mm, radial construction, on a 16 inch rim. Overall diameter comes to 631.9 mm.
About three per cent of the original overall diameter, by convention rather than by law in most places. Beyond that the speedometer error stops being trivial and clearance starts to matter.
By design and by regulation. UNECE Regulation 39 says a speedometer must never read below true speed, and may read up to ten per cent over plus 4 km/h. Manufacturers build in optimism so tyre wear never pushes them under the floor.
At a true 100 km/h the indicated speed may legally sit anywhere between 100 and 114. Most cars are set up to read two to five per cent high when new.
Yes, and by more than people expect. Going from 8 mm of tread to the 1.6 mm legal minimum takes 12.8 mm off the diameter, about two per cent, so a worn set makes an already optimistic speedometer read higher still.
It is the geometric one, π times the diameter. A loaded tyre squashes, so the real distance per revolution is two to three per cent less. Comparing two sizes with it is fine; calibrating against measured distance is not.
Not if it is done properly. Plus-sizing pairs a larger rim with a lower profile so the overall diameter barely moves, which is the whole point of the exercise.
Yes. A larger diameter covers more ground per revolution, so the odometer under-reads, service intervals arrive later than they should, and the mileage on a resale is lower than the car has actually done.
The widest point of the inflated, unloaded tyre on its specified measuring rim, sidewall bulge and moulded lettering included. The tread contact patch is narrower.
Because the aspect ratio is a percentage of the width. At 55 per cent, a 215 has a 118.25 mm sidewall against the 205’s 112.75, so the diameter grows by 11 mm even though nothing about the wheel changed.
The number is a coded maximum load per tyre and the letter a maximum speed. Both must at least match what the vehicle was approved with, and neither is part of the size arithmetic here.
The P marks a P-metric tyre built to the American standard, which is rated at a lower reference pressure than the Euro-metric equivalent. The dimensions match; the load-carrying figures do not.
Not reliably on a road car. A wider tyre changes the contact patch shape, adds steering weight and unsprung mass, and generally worsens aquaplaning resistance because there is more water to move per unit of length.
A narrower tyre at a taller profile keeps the same overall diameter while concentrating the load into a smaller patch, which cuts through snow rather than floating on it.
They read wheel speed, so a diameter change shifts every threshold they work from. Small changes are absorbed; a large one can leave a system intervening at the wrong moment without ever showing a warning.
Many cars are designed that way from the factory. Mixing sizes on the same axle is a different matter and is a failure item in most inspection regimes.
Half the diameter change, since only the bottom half sits between the axle and the road. The 11 mm gained by moving from 205/55 to 215/55 lifts the car by 5.5 mm.