Tractor Tire Conversion Chart: Standard, Radial and Metric

Agricultural tyres are sold under three notations for the same tyre. Standard sizes use an X, radials use an R, and metric sizes replace the inch width with a millimetre width and add a series number. The conversion is a fixed mapping of nominal widths, and the table below gives every common size in all three.

How the three notations relate

A standard size like 16.9X38 gives the nominal section width in inches and the rim diameter in inches. The radial version of the same tyre is written 16.9R38 — same dimensions, radial casing. The metric version, 420/85R38, replaces the inch width with the equivalent millimetre width and states the aspect ratio explicitly.

The 85 is the part worth understanding. It is the sidewall height as a percentage of the section width, and it is what makes the metric name equivalent rather than merely similar: an 85 series metric tyre has the same proportions as the old standard size, so it stands the same height on the same rim.

The millimetre widths are not conversions. 16.9 inches is 429 mm, and the metric size is called 420; 11.2 inches is 285 mm and the metric size is called 280. Those are the standard bead widths the industry settled on, which is why the conversion is a lookup rather than a multiplication.

Standard to radial to metric

Same height, different width

Lower series numbers put a wider tyre on the same rim at the same overall height. A 70 series or 65 series tyre in place of an 85 series gives more footprint and a lower ground pressure without changing the rolling diameter — which matters on four-wheel drive, where front and rear must stay in their designed ratio or the drivetrain fights itself.

The alternatives below are as published by a tyre manufacturer. Each row is a set of sizes that stand at the same height.

Method and precision

The standard-to-metric mapping is the published set of nominal widths — 9.5 to 250, 11.2 to 280, 12.4 to 320, 13.6 to 340, 14.9 to 380, 16.9 to 420, 18.4 to 460 and 20.8 to 520 — and every generated row is checked against a manufacturer's own conversion table before this page is allowed to publish. One correction: the chart in wide circulation, including the version previously on this page, gives 18.4X26 as 420/85R26. Every other 18.4 row in the same chart maps to 460, and so does the manufacturer table, so that row is an error and is not reproduced here. Sources: Vredestein Radial Tractor Tyre Conversion Table (manufacturer publication). Ken Jones Tires standard-to-metric tractor tyre conversion table.

Changing size on a working tractor

Two things decide whether a substitution is safe. Load rating first: the ply rating or load index has to meet the axle load with ballast and implement weight included, not the bare tractor. Then rolling circumference, which on four-wheel drive has to keep the front-to-rear ratio the manufacturer specified. Get that ratio wrong and the axles scrub against each other, which shows up as tyre wear long before it shows up as a drivetrain fault.

Ballast changes the answer too. A tyre carrying fluid ballast is carrying it at every moment, including on the road, and a size chosen on the bare axle load will be working harder than intended.

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