Tractor Tire Tools & Calculators

Use these tractor tire tools to decode a size, check a diameter, set ballast and pick a tread before you buy.

Tire math is where good tractors go to get confused. Our six tools do the sums for you, so you spend your time in the field instead of with a calculator. Each one answers one question. Here is what they do, with a real example for each.

Six tools, one job each

What does this tire size mean?

The size converter decodes a size in any common format. Type 710/70R42 and you get a section width of 710 mm (28 in), a 70% aspect ratio, a sidewall of 497 mm (19.6 in), radial construction and a 42 in rim. The overall diameter comes out at 81.1 in (2,061 mm). It also knows that 18.4x38 and 18.4-38 are the same tire.

How tall is the tire, and how far does it roll?

The diameter calculator gives overall diameter and rolling circumference. For 710/70R42, the diameter is 81.1 in (2,061 mm). The theoretical circumference is about 254.9 in (6.47 m, 21.2 ft). Real rolling distance under load runs 2 to 5% less, because the tire flexes where it meets the ground. Use it to compare a replacement with your original.

Will new tires upset my MFWD lead?

The lead-lag calculator compares your front and rear sizes with the new ones. Say the original is 480/70R30 front and 600/70R42 rear, and you want 650/65R42 on the rear. The diameters are 56.5 in, 75.1 in and 75.3 in. The rear to front ratio moves from 1.33 to 1.333. That is about 0.26 points more lead. Your manual tells you what range is acceptable.

Which tread do I need?

The tread type picker asks a few questions about your machine and ground, then names a code: R1, R1W, R3, R4, F-1, F-2 or I-1. A rear drive tire on wet or firm soil points to R1W, the deeper lug. Read R1 vs R1W or the tread types guide for the reasoning.

How much weight and speed can it take?

The load index and speed rating chart turns a sidewall marking into numbers. Type 155A8 and you get 3,875 kg (8,543 lb) of load at up to 40 km/h (25 mph). Give it a dual marking like 163D/166A8 and it reads both.

How much ballast should I carry?

The ballast estimator uses published rules of thumb. For a 150 PTO hp MFWD tractor pulling a towed implement, it returns a target operating weight of 18,000 to 21,750 lb (8,165 to 9,866 kg). The front axle target is 6,300 to 7,613 lb at 35%. The rear is 11,700 to 14,138 lb at 65%. It is an estimate, and your operator's manual wins.

Which tool should you use first?

• Got a size on a sidewall and no idea what it means? Start with the size converter.
• Replacing a tire with a different size? Check the diameter calculator, then the lead-lag calculator if your tractor is MFWD.
• Not sure what tread to order? Use the tread type picker.
• Reading a load or speed marking? Use the load index chart.
• Slipping or running light? Try the ballast estimator before you buy a tire.

What happens after you have your size?

Look it up in our tire listings or on the tire sizes page. Then send it to us for a quote. We are based in Bangkok, ship to the USA and Europe in about 1 week, and take payment by Zelle. For the full size story, read tractor tire sizes explained.

Send us your size on WhatsApp. We will do the rest of the math. Our idea of a fun Friday is a size conversion table, so you are in good hands.