Quick answer
A tractor size like 18.4R38 means a tyre about 18.4 inches wide, of radial construction, for a 38-inch rim. If there is a dash instead of an R (18.4-38) the tyre is bias-ply. Metric sizes such as 460/85R38 give the width in millimetres and the sidewall height as a percentage, and 460/85R38 is the metric equal of 18.4R38. Rear tyres normally use an R-1 lug tread and two-wheel-drive front tyres an F-2 ribbed tread.
Reading an inch size: 18.4R38
| Part | Meaning |
|---|---|
| 18.4 | Section width in inches (about 467 mm) |
| R or - | R means radial. A dash (18.4-38) means bias-ply |
| 38 | Rim diameter in inches |
| 8PR / 141A8 (example) | Ply rating, or load index with a speed symbol such as A8 (40 km/h) |
Inch sizes come from older tyre standards, but they are still the sizes most tractors in Zimbabwe use. Their sidewall height is roughly 85% of their width.
Reading a metric size: 460/85R38
Newer tractors and replacement tyres often use metric sizes:
- 460 is the width in millimetres.
- 85 is the aspect ratio: the sidewall is 85% of the width.
- R38 means radial, for a 38-inch rim.
Lower aspect ratios such as 480/70R28 or 710/70R38 are wider and lower, spreading weight over more ground to reduce soil compaction.
Common tractor tyre sizes and their equivalents
This chart lists sizes we are often asked for. The metric figure in the second column is the usual 85-series equivalent with a similar diameter. Always check rim width and clearance before changing to a different size.
| Inch size | Metric equivalent | Typical position and use |
|---|---|---|
| 18.4R42 | 460/85R42 | Rear, large row-crop tractors |
| 18.4R38 | 460/85R38 | Rear, large 2WD and 4WD tractors |
| 18.4R30 | 460/85R30 | Rear on mid-size tractors, front on large 4WD |
| 16.9R30 | 420/85R30 | Rear on mid-size tractors |
| 16.9-28 | 420/85-28 | Rear on smaller tractors, TLB rear |
| 14.9R28 | 380/85R28 | Rear on smaller tractors, front on 4WD |
| 13.6R28 | 340/85R28 | Rear on compact tractors, front on 4WD |
| 12.4-28 | 320/85-28 | Rear on compact tractors |
| 11.2R24 | 280/85R24 | Front on 4WD tractors |
| 9.5R20 | - | Front on smaller 4WD tractors |
| 7.50-16 | - | Front on 2WD tractors (F-2 tread) |
Tread types: R-1, F-2 and others
| Code | Tread | Use |
|---|---|---|
| R-1 | Standard angled lugs | Rear and 4WD front tyres for general field work |
| R-1W | Deeper, wider lugs | Wet and heavy soils |
| R-2 | Extra-deep lugs | Very wet conditions such as rice or sugar cane |
| R-4 | Shallower industrial lugs | TLBs and tractors on hard or paved ground |
| F-2 | Ribbed front tread | Front tyres on 2WD tractors, for steering |
Fit R-1 tyres with the lugs pointing the right way: seen from above, the point of the "V" should face the direction of forward travel. Tyres fitted backwards lose traction and wear fast.
4WD tractors: keep the front and rear in step
On a four-wheel-drive tractor, the front and rear tyres are a matched set. The front axle is geared to turn slightly faster than the rear, a difference called lead, and the tyre sizes are chosen to keep it within the maker's range. If you change the rear size, you may need to change the front too, or the drivetrain will fight itself, wasting fuel and wearing tyres. Replace rear tyres as a pair and front tyres as a pair.
Radial or bias?
- Radial tractor tyres have flexible sidewalls and a longer, flatter footprint. They give better traction, less wheel slip and less soil compaction, and they usually last longer.
- Bias tyres cost less and have stiff, tough sidewalls. They suit lighter work, older tractors and implements.
Pressure matters even more for radials. See our guide to agricultural tyre pressure.
How to order the right tractor tyre
- Read the full size and tread code from the sidewall, for example "18.4R38 R-1".
- Note whether it is front or rear, and whether the tractor is 2WD or 4WD.
- Tell us the tractor make and model, such as Massey Ferguson, John Deere or New Holland.
- Say how many you need (usually two) and where the farm is.
Tractor tyres from Proclassic
Proclassic has supplied tyres to Zimbabwean agriculture since 1996. We carry the Galaxy agricultural range for tractors, trailers and implements, including 18.4R38, 18.4R30, 14.9R28, 13.6R28, 12.4-28, 11.2R24 and 7.50-16 F-2, plus wide metric sizes such as 480/70R28 and 710/70R38. See all sizes on our agricultural tyres page, or send your size on WhatsApp for stock and price.
Get a price on WhatsApp
Send us these four details and we will reply with stock and today’s price:
- Tyre size, from the sidewall
- Quantity — e.g. 2 rear, 2 front
- Vehicle or equipment, make and model
- Delivery location, or collect in Harare or Mutare
Or call +263 716 194 485 · info@proclassicgroup.com