Tire Size Explained: Diameter, Circumference and Alternative Sizes

Why your tire size is worth a closer look

New wheels, cheaper winter tires or simply a different look: sooner or later almost every driver wonders whether a size other than the original would fit. The answer depends on just a few numbers. Once you understand them, you can work out in a minute how big a tire really is, how far it deviates from the original and what that means for the speedometer, ground clearance and space in the wheel arch.

You can recalculate every example in this guide with the tire size calculator. It shows the old and new sizes side by side, sketched to scale, with the speedometer error and wheel geometry.

Decoding the tire size: 205/55 R16

Every sidewall carries a size designation to a European standard. Taking 205/55 R16 91V as an example:

A detailed guide with tables for the load index and speed rating is in the article How to read your tire size.

Calculating diameter and circumference

These three numbers give you the overall diameter. It consists of the rim plus the sidewall height twice, once at the top and once at the bottom:

Sidewall height = width × aspect ratio ÷ 100

Diameter = inches × 25.4 + 2 × sidewall height

Rolling circumference = diameter × π

For 205/55 R16 that means: the sidewall is 205 × 0.55 = 112.75 mm tall. The rim measures 16 × 25.4 = 406.4 mm. The diameter is 406.4 + 2 × 112.75 = 631.9 mm. Multiplied by π, that gives a circumference of about 1,985 mm. So the wheel turns roughly 504 times per kilometre.

This calculated value is the nominal circumference. The actual rolling circumference while driving is slightly smaller because the tire deflects under load. It also depends on pressure, tread depth and speed. For comparing two sizes this hardly matters, as both tires deflect in a similar way. What counts is the percentage difference.

Alternative sizes: wider, lower, bigger

The most popular change is the plus size: one inch more rim, a lower sidewall and often a slightly wider tire. The aim is to keep the overall diameter as close as possible to the original. The classic example is switching from 205/55 R16 to 225/45 R17:

The first three alternatives are close to the original. 235/35 R19, however, is almost 2.5% bigger and clearly outside the usual rule of thumb. The principle also works the other way round as a minus size: many drivers fit rims one inch smaller with a taller sidewall for winter. That is often cheaper, and the thicker sidewall protects the rim better.

How much deviation is acceptable?

There is no fixed legal percentage for tire diameter in Germany. The tire trade uses a rule of thumb: the new size's rolling circumference should be at most about 1.5% larger and at most about 2% smaller than that of the original tires. Some sources give ±3% as the outer limit. The calculator shows green within the narrow rule of thumb, amber up to ±3% and red beyond that.

Why is the upper limit tighter? Because of the speedometer. A larger tire covers more distance per revolution, so the speedometer shows less than your real speed. Under UN Regulation No. 39 (ECE R39), however, a speedometer must never show less than the true speed. Factory speedometers therefore usually overread a little. A larger tire must not use up this margin. More on this in Speedometer error from different tires.

A smaller tire is uncritical for the speedometer. It does reduce ground clearance, though, and makes the odometer and consumption display overread. With large deviations, ABS, ESP and driver-assistance systems can also receive wrong wheel speeds.

What is legally allowed? CoC, type approval and inspection

The rule of thumb only tells you what is technically sensible. Which size you may actually fit is determined by your papers. In Germany the registration certificate (Zulassungsbescheinigung Teil I) usually lists just one tire size. All sizes approved by the manufacturer are in the EC certificate of conformity (CoC). If your preferred size is listed there, you may fit it without further steps, provided the rim, load index and speed rating match too.

If it is not listed, you need proof. For wheels there are two routes: a general type approval (ABE) under § 22 StVZO applies without an inspection; you only have to carry it if the document says so. A parts report (Teilegutachten), on the other hand, requires an inspection by an approved expert without delay (§ 19(3) StVZO). Without either, the vehicle's operating permit can lapse because a hazard is to be expected (§ 19(2) StVZO). That has consequences: a fine at a roadside check and possible problems with your insurer after an accident.

Tires themselves carry a UN approval: ECE R30 for car tires and ECE R54 for commercial vehicle tires. In addition, ECE R117 covers rolling noise, wet grip and rolling resistance. You can recognise the approval by the E mark in a circle on the sidewall.

Wheels: rim width and offset

With new wheels, two further values come into play. The rim width, e.g. 7J, is given in inches. The offset (ET, from the German Einpresstiefe) is the distance between the wheel's centre line and the face that sits against the hub. A high offset pulls the wheel inwards, a low one pushes it outwards.

This has three effects. The track width changes per axle by twice the offset difference: from ET 45 to ET 35 the track grows by 20 mm. The outer rim edge moves out by half the extra width plus the offset reduction. The inner rim edge moves in by half the extra width minus the offset reduction, towards the strut and brake. Example: from 6.5J ET46 to 7.5J ET45 the rim sticks out 13.7 mm further on the outside and sits 11.7 mm closer to the suspension on the inside.

German law requires the wheels to remain adequately covered (§ 36a StVZO). A few millimetres can decide whether a tire rubs on the wheel arch or the strut. That is why parts reports for wheels often come with conditions, such as rolling the wheel-arch lip.

Checklist for changing sizes

Don't forget load index and speed rating

A different size often has a different load index too. The new tire must reach at least the value stated in the papers. This matters especially for estate cars, vans and vehicles with a tow bar. Low-profile tires are therefore often available as reinforced versions marked XL or Reinforced.

The same goes for the speed rating: at least the registered value. Only winter tires may have a lower rating. In that case a notice of the permitted maximum speed must be displayed in the driver's field of view.

Tire age and condition

Even the right size is no help if the tire is too old. How to read the age from the DOT code and when experts recommend replacing tires is explained in Tire age from the DOT code. For the cold season, take a look at the winter tire rules: since October 2024 only the Alpine symbol counts in Germany.

Conclusion

A tire size is not a secret code. With width, aspect ratio and rim size you can calculate diameter and circumference in seconds. If the new size is between −2% and +1.5%, it is usually technically fine. Legally, though, only what the CoC, type approval or parts report approves for your car counts. Check your preferred size in the tire size calculator before buying tires or wheels. And while you are at it: the fuel cost calculator shows what wider tires might cost you at the pump.