How to Read Tractor Tyre Sizes: An NZ Guide (520/70R38 vs 16.9-38 Explained)

Every tractor tyre has its size moulded into the sidewall, but the two main formats look nothing alike. One tyre reads 520/70R38, the next reads 16.9-38, and you can find both on the same farm.

In short: a metric size like 520/70R38 gives the width in millimetres, the sidewall height as a percentage of that width, the construction (R for radial) and the rim diameter in inches. A standard size like 16.9-38 gives the width in inches and the rim diameter, and the dash means bias ply. The two are not the same size. A 520/70R38 is about 90 mm wider and matches an 18.4R38 in conversion charts.

Here's how to read tractor tyre sizes, and what to check before you buy from our used agricultural wheels range or anywhere else.

The Two Tractor Tyre Size Formats in NZ

Most tractor tyres in New Zealand carry one of two size formats:

• Metric sizing, such as 520/70R38. Common on modern radial tractor tyres.

• Standard (inch) sizing, such as 16.9-38 or 16.9R38. Still found on older tractors and on plenty of used tyres in circulation.

Both formats give you width and rim diameter. Only the metric format tells you the sidewall height.

How to Read a Metric Tractor Tyre Size (520/70R38)

Marking	What it means	For a 520/70R38
520	Section width in millimetres	520 mm wide
70	Aspect ratio: sidewall height as a percentage of width	Sidewall about 364 mm (70% of 520)
R	Radial construction	Radial
38	Rim diameter in inches	Suits a 38 inch rim

The aspect ratio is the sidewall height shown as a percentage of the tyre's width. A lower number means a shorter sidewall. A 520/65 has a shorter sidewall than a 520/70, even though both are 520 mm wide.

How to Read Standard Tractor Tyre Sizes (16.9-38)

Marking	What it means	For a 16.9-38
16.9	Section width in inches	About 429 mm wide
Dash (-)	Bias (diagonal ply) construction	Bias ply
38	Rim diameter in inches	Suits a 38 inch rim

Radial vs bias: if the dash is replaced with an R (16.9R38), the tyre is radial. On some older tyres you'll see an x between the numbers (16.9x38) instead of a dash, which also means bias ply. A 16.9-38 and a 16.9R38 share a nominal size but are built differently. Don't mix the two on one axle.

Standard sizes don't show an aspect ratio. Most inch sizes match an 85 series metric size. A 16.9R38, for example, is equivalent to a 420/85R38, and some tyres carry both markings.

520/70R38 vs 16.9-38: Are They the Same Size?

No. A 520/70R38 is a bigger tyre than a 16.9-38. In tyre makers' equivalence charts, 520/70R38 is grouped with 18.4R38, while 16.9R38 lines up with 480/70R38. On width alone, you're comparing 520 mm with roughly 429 mm.

Tractor Tyre Size Conversion Chart

Standard size	85 series	70 series	65 series
12.4R28	320/85R28	360/70R28	420/65R28
16.9R38	420/85R38	480/70R38	540/65R38
18.4R38	460/85R38	520/70R38	600/65R38

The chart lists radial sizes. A bias 16.9-38 has the same nominal size as a 16.9R38, but if you move from bias to radial, change both tyres on that axle. Treat any conversion as a guide only. Rolling circumference can differ slightly between brands and tread patterns, even at the same size. Measure or check with the maker before relying on it.

Can I Fit a 520/70R38 in Place of a 16.9-38?

Possibly, but check three things first:

• Rim width. A wider tyre may need a wider rim. Check the approved rim widths for the new tyre size before fitting.

• Clearance. Check the gap to mudguards, cab steps and linkage. For front tyres, check it again at full steering lock.

• Front to rear ratio on a 4WD tractor. Going bigger on the rear changes the ratio between front and rear tyres, which the drivetrain is set up for. Check any new combination with your dealer or tyre specialist.

Always confirm a size change against your tractor manufacturer's approved tyre list.

Other Tractor Tyre Markings to Check

Size is the starting point. These markings matter too:

Load index and speed symbol

The number is a reference for the load the tyre can carry at a set pressure and speed. The letter is the maximum speed the tyre is rated for when loaded. Make sure both meet your tractor's requirements.

Ply rating (PR)

Older bias tyres often show a ply rating instead of, or alongside, a load index.

Date code

A four digit code showing the week, then the year of manufacture. For example, 3214 means the 32nd week of 2014. On a used tyre, weigh age alongside tread depth and sidewall condition.

Tubeless or tube type

Look for TL (tubeless) or TT (tube type) on the sidewall.

Matching Tyres Across an Axle

• Run the same size and construction on both sides of an axle.

• If you're only replacing one tyre, match the tread wear on the other side as closely as you can.

• On a 4WD tractor, keep the front and rear sizes within the combination your manufacturer approves.

Finding Used Tractor Tyres in NZ by Size

Our used agricultural wheels and tyres are listed by size, brand and tread remaining. As at September 2026, stock included a Pirelli 520/70R38, a Continental 520/70R38 dual set, a Michelin 480/65R28 and Snaplok R38 rims, listed to fit 16.9R38 tyres.

What's listed changes regularly, and our team can often source sizes that aren't online. We're based in Southland and deliver nationwide, with international shipping available.

What to Send Us

To match a tyre, send us:

• A clear photo of the sidewall markings (clean off mud first and shoot side on in daylight)

• Tractor make, model and year

• Front and rear tyre sizes

• Rim size, if you can read it on the rim

• Tyres only, or tyres on rims

• Your town, for a freight quote

Call us on 0800 423 131 or send the details through our Find A Part form. We'll let you know what we have in stock or can track down.