Question 1

What is the metric equivalent of an 18.9-30 tractor tyre?

Accepted Answer

The closest metric equivalent to an 18.9-30 tractor tyre is 480/70R30. The overall diameter of the original 18.9-30 is approximately 1,381 mm, and the 480/70R30 has an overall diameter of approximately 1,388 mm — a difference of under 0.5%. The 480/65R30 is also used as a replacement and gives a slightly lower overall diameter of around 1,350 mm.

Question 2

Can I fit a metric tyre on an old inch rim?

Accepted Answer

Yes — the rim diameter in inches is the same in both old and new notation. A 30-inch rim is a 30-inch rim regardless of whether the tyre uses inch or metric sizing. What you must check is the rim width: each metric tyre section width has a permitted rim width range. For example, a 480 mm section width tyre requires a rim between 14 and 16 inches wide. If your rim falls within the permitted range for the new tyre, it will fit.

Question 3

Does a metric tractor tyre have the same overall diameter as the inch tyre it replaces?

Accepted Answer

Very nearly, for the standard equivalents. The industry-standard metric replacements for common inch sizes are chosen to give the closest possible overall diameter — typically within 0.5–1.5%. A larger difference will affect your tractor's road speed, speedometer accuracy, and may cause clearance problems with mudguards or front axle housings. Use this calculator to check the OD comparison before buying.

Question 4

What does the aspect ratio mean on a tractor tyre?

Accepted Answer

The aspect ratio (the middle number in metric notation, e.g. 70 in 480/70R30) is the section height expressed as a percentage of the section width. A 480/70R30 tyre has a section height of 480 × 0.70 = 336 mm. Old cross-ply inch tyres had an aspect ratio of approximately 85–90%. Modern replacements typically use 65R or 70R — 70R is the closest match to old cross-ply proportions for rear tractor applications.

Question 5

What is the difference between cross-ply and radial tractor tyres?

Accepted Answer

Cross-ply (bias-ply) tyres have cords running diagonally across the tyre at an angle of 30–40 degrees. Radial tyres have cords running perpendicular to the direction of travel with a separate belt layer. Radial tyres are more flexible, run cooler, have better fuel efficiency, give more even footprint pressure (better for soil compaction), and last longer. Almost all new agricultural tyres are radial. When replacing old cross-ply tyres with radials, fit radials on both wheels of the same axle — never mix cross-ply and radial on the same axle.

Question 6

How do I find the rim width on my tractor?

Accepted Answer

The rim width is stamped on the rim itself — look on the inside face of the rim, near the centre. It will show a size like "15×30" (15-inch wide rim, 30-inch diameter) or sometimes just the width in inches. If you cannot find the stamp, measure the internal width of the rim between the bead seats with a tape measure. This measurement in inches is your rim width.

Question 7

Should I fit IF or VF tyres when replacing old inch tyres?

Accepted Answer

IF (Improved Flexion) and VF (Very High Flexion) are modern tyre technologies that allow the tyre to carry the same load at significantly lower inflation pressure, or a higher load at normal pressure. They are worth considering when replacing old tyres if you want to reduce soil compaction — a key benefit on modern arable farms. IF tyres can carry the same load at 20% lower pressure than standard tyres; VF tyres at 40% lower pressure. They are more expensive but give real agronomic benefits. Not all old rims are suitable — check with the tyre supplier.

Old Size	Metric Equivalent	OD Old (mm)	OD Metric (mm)	Diff
9.5-24	270/95R24	931	935	+0.4%
11.2-28	300/95R28	1,072	1,080	+0.7%
12.4-28	320/90R28	1,096	1,101	+0.5%
14.9-28	380/85R28	1,202	1,213	+0.9%
16.9-30	420/85R30	1,296	1,311	+1.2%
16.9-34	420/85R34	1,398	1,413	+1.1%
18.4-30	460/85R30	1,351	1,361	+0.7%
18.9-30	480/70R30	1,381	1,388	+0.5%
18.9-34	480/70R34	1,483	1,490	+0.5%
20.8-38	520/85R38	1,590	1,613	+1.4%
23.1-34	580/70R34	1,619	1,624	+0.3%
24.5-32	620/75R32	1,637	1,638	+0.1%
28.1-26	710/70R26	1,682	1,684	+0.1%
30.5-32	800/65R32	1,890	1,882	-0.4%

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