205/60R18 vs 265/45R18

265 is the section width in millimetres, 65 is the aspect ratio (sidewall height as a percentage of the width), R means radial construction, and 17 is the rim diameter in inches. So 265/65R17 fits a 17-inch rim, is 265 mm wide and has a sidewall about 172 mm tall.

As a rule of thumb the overall diameter should stay within about 3% of the original, and the rim diameter must match. TyreCalculator shows the diameter difference as a percentage and flags it green (within 3%), amber (3–5%) or red (over 5%, or a different rim size).

Keeping the overall diameter within about ±3% of the original keeps your speedometer, odometer, gearing and ABS/traction systems close to their factory calibration. Beyond that those readings and behaviours drift, which is why the verdict turns amber and then red.

Yes. A larger overall diameter covers more distance per revolution, so your speedometer reads slower than you are actually travelling. TyreCalculator shows this directly — for example "reads 100, doing 107" — based on the diameter difference between the two sizes.

Often, within limits. Each rim width suits a range of tyre widths, and a wider tyre can foul suspension or bodywork. TyreCalculator shows the section-width difference so you can judge the change, but always check the physical clearance on your own vehicle. Clearance is not the only limit: the Tire and Rim Association notes that the load and cold inflation pressure put on a rim and wheel must not exceed the rim and wheel manufacturer's recommendations even where the tyre is approved for more. Some rims and wheels are stamped with a maximum load and pressure; where they are not, the wheel maker is the one to ask. A load index tells you what the tyre can carry, not what the wheel under it can.

Car, van and light-truck sizes (265/65R17, LT265/70R17, 215/70R16C), flotation sizes (33x12.50R15), metric motorcycle sizes (120/70ZR17), truck metric (315/80R22.5), agricultural, earthmover and industrial metric (480/70R38, 380/85R34, 750/65R25), and bicycle sizes in ISO/ETRTO or French form (37-622, 700x35C). Inch-denominated sizes such as 11R22.5, 12.4-28 and 4.00-18 are recognised and explained but not measured: their overall diameters come from ETRTO and TRA lookup tables rather than from the size itself, and an estimate would be out by more than the 3% a fitment decision turns on.

The ISO/ETRTO size is two numbers, such as 37-622: 37 is the tyre width in millimetres and 622 is the bead seat diameter of the rim, also in millimetres. The rim must have that same bead seat — the inch name on the wheel will not tell you, because a "26 inch" wheel can be 559, 571 or 590 mm. The French designation (700x35C) is the same wheel written differently: 700C is always a 622 mm bead seat.

It is the sheet that comes with every new car, van and truck tyre sold in Europe, compulsory since May 2021 under Regulation (EU) 2020/740. It grades three things: fuel efficiency (rolling resistance) from A to E, wet grip (braking distance on a wet surface) from A to E, and external rolling noise as a measured value in decibels plus a class from A to C. Two pictograms may also appear — the three-peak mountain snowflake for severe snow, and the ice grip mark, which only a car tyre can carry. None of it is moulded into the sidewall: the label is on the tyre in the shop, in the seller’s listing, and in the EU’s public register behind the QR code. Grades only compare between tyres of the same class, so a truck tyre’s B is not a car tyre’s B.

Overall diameter is the full height of the inflated tyre; rolling circumference is the distance it travels in one revolution (π × diameter). Together they set your effective gearing and the revolutions-per-kilometre figure TyreCalculator calculates.