What is 18.9-30 in metric?

The metric equivalent of 18.9-30 is 480/70R30. The 480 refers to the section width in millimetres (18.9 inches × 25.4 ≈ 480 mm), the 70 is the aspect ratio (the tyre's height is 70% of the section width), R means radial construction, and 30 is the rim diameter in inches — unchanged from the imperial size. The overall diameter of 480/70R30 is within 0.5% of the original 18.9-30, making it an excellent match.

What is 16.9-34 in metric?

The metric equivalent of 16.9-34 is 420/85R34. The 420 mm section width corresponds to 16.9 inches, the 85 aspect ratio reflects the older taller cross-ply profile, and R34 means radial on a 34-inch rim. The overall diameter is approximately 1.1% larger than the original — within the safe ±3% tolerance for most 4WD drivetrains. This is the standard rear tyre for Ford 7600, MF 590, and many similar 80s-era tractors.

What is 16.9-30 in metric?

The metric equivalent of 16.9-30 is 420/85R30. The section width converts from 16.9 inches to approximately 420 mm, aspect ratio 85 gives the similar overall profile to the original cross-ply tyre, on a 30-inch rim. The overall diameter difference is approximately +1.2% — within tolerance. Common on MF 390, Ford 6600, and similar mid-80s tractors.

What is 14.9-28 in metric?

The metric equivalent of 14.9-28 is 380/85R28. Section width of 14.9 inches converts to approximately 380 mm, aspect ratio 85 maintains the tall profile of the original cross-ply, on a 28-inch rim. The overall diameter is within 0.9% of the original — an excellent match. This is a common front wheel size on many 4WD tractors including MF 390T and JD 3040.

What does the R mean in a tractor tyre size?

R stands for Radial construction. In a radial tyre, the internal cord plies run at 90 degrees (radially) from the bead, and a belt layer runs around the circumference underneath the tread. This allows the sidewall to flex independently of the tread area, creating a larger, flatter footprint on the ground. Radial tyres deliver lower soil compaction, better traction, reduced fuel consumption, and longer service life compared to cross-ply (bias-ply) tyres. If there is no R in the size, the tyre is cross-ply — for example 16.9-34 (cross-ply) versus 420/85R34 (radial).

How do I know if my tractor rim is compatible with a metric tyre?

Look for a stamp on the inside face of the rim, usually near the centre — it will show something like 15×30 or W15×30, meaning the rim is 15 inches wide and fits a 30-inch tyre. Check the tyre manufacturer's fitment chart for the minimum and maximum rim widths for your chosen tyre. For example, a 420/85R34 tyre typically requires a rim of 13–15 inches wide. A rim that is too narrow will cause the tyre sidewall to bow outward, causing rapid wear and potential bead failure. Never guess — the rim width stamp is always present on correctly marked agricultural rims.

Can I mix cross-ply and radial tyres on my tractor?

No — never mix cross-ply and radial tyres on the same axle. The two construction types flex very differently under load. Cross-ply tyres are stiffer and have a more rounded footprint; radials flex more and create a wider footprint. Mixing them on the same axle causes unequal load distribution, unpredictable handling, and accelerated wear on both tyres. If you are replacing one tyre on an axle, you must fit the same construction type as the existing tyre — or replace both. Many tyre manufacturers explicitly state that warranty is void if tyres are mixed on the same axle.

How much diameter difference is acceptable when converting from inch to metric?

The general industry standard is ±3% overall diameter difference. For 4WD tractors this is particularly important because the front and rear axles must maintain a precise speed ratio (called axle lead or lag). If the rear tyres are significantly larger than specified, the front axle will be dragged by the rear, causing tyre scrub, increased fuel consumption, and potential damage to the 4WD coupling. Most metric equivalents of imperial agricultural tyres are within 1–2% of the original diameter — well within tolerance. Use the calculator on this page to check the exact figure for your size.

What is the difference between 70 and 85 aspect ratio tractor tyres?

The aspect ratio is the tyre's height expressed as a percentage of its section width. An 85 series tyre (e.g. 420/85R34) is taller relative to its width — this mirrors the profile of old cross-ply tyres and is the common direct replacement for imperial sizes. A 70 series tyre (e.g. 480/70R34) is lower profile — wider but shallower. The 70 series gives better stability and a slightly wider footprint at the cost of a smaller overall diameter. Many modern tractors specify 70 series as original equipment. When converting old imperial tyres, check which metric replacement gives a diameter closest to the original — sometimes 85 series, sometimes 70 series.

Machinery & Maintenance

How to Convert Tractor Tyre Sizes: Inch to Metric UK Guide

21 common conversions including 18.9-30, 16.9-34, 14.9-28, 20.8-38 — with rim width guide, aspect ratio explained, and brand tyre specs for Massey Ferguson, John Deere, Ford, New Holland and Case IH.

Published 19 May 2026 · 8 min read

If you have replaced tyres on an older British or European tractor, you have almost certainly faced the same problem — the size stamped on the sidewall is in old imperial notation (18.9-30, 16.9-34, 14.9-28) and modern tyre catalogues list everything in metric (480/70R30, 420/85R34). They look completely different, but most have direct equivalents. The key is knowing which metric tyre is a safe match and which will cause problems with your 4WD drivetrain or rim fitment.

This guide explains how to read both systems, gives a complete conversion table for 21 common sizes, covers rim width compatibility, and answers the most-searched questions about specific sizes.

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How to Read Tractor Tyre Sizes

Old Imperial (e.g. 16.9-34)

16.9 – 34

16.9 — nominal section width in inches
34 — rim diameter in inches
No aspect ratio stated — defaults to ~85%
No R — cross-ply (bias-ply) construction

Modern Metric (e.g. 420/85R34)

420/85R34

420 — section width in millimetres
85 — aspect ratio (height = 85% of width)
R — radial construction
34 — rim diameter in inches (unchanged)

The rim diameter in inches is the same in both systems — a 34-inch rim is always a 34-inch rim. The section width converts at roughly 25.4 mm per inch (16.9 × 25.4 = 429 mm, rounded to 420 mm in the ETRTO standard). The aspect ratio of 85 is the closest equivalent to the profile of a traditional cross-ply agricultural tyre.

Full Conversion Table — 21 Common UK Tractor Tyre Sizes

Diameter difference shown as % change in overall diameter. Within ±3% is safe for most 4WD drivetrains. Green = excellent match (<1%). Amber = good match (1–2%).

Imperial Size	Metric Equivalent	OD Diff	Rim Width (in)	Common On
9.5-24	250/85R24	+0.4%	7.5–9.0"	Small row crop tractors, vineyard
11.2-24	280/85R24	+0.6%	8.5–10.0"	Row crop front axle
11.2-28	280/85R28	+0.5%	8.5–10.0"	Standard front wheel — MF 135, Ford 4000
12.4-28	320/85R28	+0.5%	9.5–11.0"	Front wheel — Ford/New Holland, Case IH
13.6-28	340/85R28	+0.8%	10.0–12.0"	Front drive wheel — many 4WD compact tractors
14.9-28	380/85R28	+0.9%	11.0–13.0"	Front wheel — MF 390, JD 3040
13.6-36	340/85R36	+0.7%	10.0–12.0"	Rear — smaller MF and Ford models
14.9-30	380/85R30	+0.8%	11.0–13.0"	Rear — MF 135, David Brown 990
16.9-28	420/85R28	+0.6%	13.0–15.0"	Rear — JD 3040, Case 895
16.9-30	420/85R30	+1.2%	13.0–15.0"	Rear — MF 390, Ford 6600
16.9-34	420/85R34	+1.1%	13.0–15.0"	Rear — MF 590, Ford 7600, NH 60-series
18.4-30	460/85R30	+0.9%	14.0–16.0"	Rear — JD 3140, Case IH 956
18.4-34	460/85R34	+1.0%	14.0–16.0"	Rear — MF 690, Ford 8000 series
18.9-30	480/70R30	+0.5%	14.0–16.0"	Rear — JD 6030, Ford TW-series, Case IH
18.4-38	460/85R38	+1.1%	14.0–16.0"	Rear — NH 70-series, MF 3000 series
20.8-38	520/85R38	+1.4%	16.0–18.0"	Rear — JD 6R/7R, Case IH Puma, NH T6/T7
20.8-42	520/85R42	+1.3%	16.0–18.0"	Rear — larger MF 7000 series
23.1-30	580/70R30	+0.8%	17.0–19.0"	Rear — JD 7020, Case IH Magnum (older)
23.1-34	580/70R34	+0.7%	17.0–19.0"	Rear — JD 7R 210, Case IH Puma 185
24.5-32	620/70R42	+1.1%	18.0–20.0"	Rear — high horsepower 4WD
30.5L-32	800/70R32	+0.9%	23.0–26.0"	Rear — JD 8000 series, Case IH Steiger

Diameter differences are approximate. Always verify with tyre manufacturer datasheets before purchasing. Use the calculator for precise OD calculations.

85 Series vs 70 Series — Which Do I Need?

When looking up metric equivalents you will see two main aspect ratio options for most sizes — 85 series (e.g. 420/85R34) and 70 series (e.g. 480/70R34). They are not interchangeable.

85 Series — Direct Cross-Ply Replacement

• Taller, narrower profile — closest to old imperial cross-ply tyre dimensions
• Best choice when replacing imperial tyres like 16.9-34 or 18.4-38
• Overall diameter closely matches the original
• Standard fitment on most tractors pre-2000

70 Series — Modern Wide-Profile Tyre

• Lower profile, wider footprint — lower soil compaction
• Standard fitment on most tractors from ~2000 onwards
• Overall diameter smaller than 85 series in same width
• Check OD carefully before fitting as direct imperial replacement

The tractor tyre converter tool calculates the exact overall diameter for both options so you can compare which is the closest match to your original.

Rim Width Compatibility

Before ordering new tyres, confirm your rim width. The stamp is on the inner face of the rim — look for a code like W15×34 or 15L-34, meaning the rim is 15 inches wide and suits a 34-inch tyre.

Too narrow a rim — the tyre sidewall bows outward, wearing the tread unevenly and risking bead failure. Never fit a tyre to a rim narrower than the manufacturer minimum.

Too wide a rim — the tyre bead is overstretched, reducing sidewall height and creating a harsh ride with less soil compaction benefit.

Rule of thumb: the rim width should be 65–85% of the tyre's section width for agricultural applications.

4WD Tractors: The ±3% Diameter Rule

On a 4WD tractor, the front and rear axles must maintain a precise speed ratio — typically the front turns 1–4% faster than the rear (called axle lead). This compensates for the front tyres covering a slightly shorter path when cornering. If your replacement tyres change the overall diameter significantly, the lead ratio shifts.

Rear tyres too large

The rear axle effectively overtakes the front, forcing the front tyres to be dragged faster than they turn. This causes tyre scuff, high fuel consumption, and heat build-up in the 4WD coupling.

Rear tyres too small

The front tyres push ahead of the rear, causing front tyre hop and scrub when cornering. Less severe than rear-too-large but still causes accelerated tyre wear.

The safe tolerance is generally ±3% change in overall diameter from the original specification. Most direct metric equivalents of imperial sizes are within 1–2% — but always check before purchasing, especially on tractors with electronic 4WD engagement.

Tyre Sizes by Tractor Brand & Model

Original equipment rear tyre sizes for common UK tractors. Sizes vary by specification — always confirm against your tractor's handbook or sidewall marking.

Tractor Model	Standard	Rear Tyres	Front Tyres
Massey Ferguson 135/165	Imperial cross-ply	12.4-28 / 14.9-28	6.00-16
Massey Ferguson 390/390T	Imperial / early metric	16.9-30 / 420/85R30	9.5-28 / 250/85R24
Massey Ferguson 6000 series	Metric radial	480/70R34 or 540/65R38	420/70R24 or 480/65R24
Ford 4000 / 5000	Imperial cross-ply	14.9-30 or 16.9-30	7.50-16 or 9.5-28
Ford 6600 / 7600	Imperial cross-ply	16.9-34	9.5-28
New Holland 60-series / TM	Imperial/metric mix	460/85R38 or 18.4-38	9.5-24 or 380/85R28
New Holland T6 / T7	Metric radial	480/70R38 or 540/65R38	420/70R24 or 480/65R24
John Deere 3040 / 3140	Imperial cross-ply	16.9-30 or 18.4-30	9.5-28 or 11.2-28
John Deere 6030 / 6R	Metric radial	480/70R34 or 540/65R38	420/70R24 or 480/65R24
Case IH 956 / 1056	Imperial/early metric	18.4-34 or 460/85R34	9.5-28 or 280/85R28
Case IH Puma / Maxxum	Metric radial	480/70R38 or 540/65R38	380/85R28 or 420/65R24
Fendt 300 / 700 / 800	Metric radial	540/65R38 or 600/65R38	440/65R24 or 480/65R24

Load Index and Speed Rating

Modern metric agricultural tyres carry a load index and speed rating after the size — for example 420/85R34 137A8. The 137 is the load index (the maximum load the tyre can carry, in kg, at the rated inflation pressure), and A8 is the speed rating (A8 = maximum 40 km/h). You must match or exceed the load index of the original tyre — never fit a lower-rated tyre on the same axle.

Common agricultural load indices

130 = 3,000 kg

133 = 3,350 kg

137 = 3,750 kg

140 = 4,000 kg

143 = 4,500 kg

146 = 5,000 kg

149 = 5,600 kg

152 = 6,300 kg

Calculate Your Exact Tyre Match

Enter any imperial size and get the exact OD difference, rim width range, and nearest metric equivalents ranked by diameter match.

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