Question 1

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

Accepted Answer

The closest metric equivalent to an 18.9-30 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%.

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. What you must check is the rim width: each metric tyre section width has a permitted rim width range. If your rim falls within the permitted range for the new tyre, it will fit.

Question 3

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

Accepted Answer

Very nearly, for the standard equivalents — typically within 0.5–1.5%. A larger difference will affect road speed and speedometer accuracy, and may cause clearance problems. Use this calculator to check the OD comparison before buying.

Question 4

What does the aspect ratio mean on a 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. Old cross-ply inch tyres had an aspect ratio of approximately 85–90%. Modern replacements typically use 65R or 70R.

Question 5

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

Accepted Answer

Cross-ply (bias-ply) tyres have cords running diagonally; radial tyres have cords running perpendicular with a separate belt layer. Radials are more flexible, run cooler, and give more even footprint pressure. When replacing old cross-ply tyres with radials, fit radials on both wheels of the same axle — never mix them.

Question 6

How do I find the rim width on my wheel?

Accepted Answer

The rim width is stamped on the rim itself — look on the inside face near the centre. It will show a size like "15×30" (15-inch wide rim, 30-inch diameter). If you cannot find the stamp, measure the internal width between the bead seats.

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%

FleetBolt

Tyre Size Converter

Enter Your Old Tyre Size

How to Read Your Tyre Sidewall

Industry-Standard Size Equivalents — Quick Reference

Frequently Asked Questions

Track Tyre Sizes Across Your Whole Fleet

We use cookies