Tractor tires are going through a growth spurt. Up until six or seven years ago, the biggest farm tire you could buy for a tractor or combine was 6.8 ft. tall, categorized as Group 48. Now, tire manufacturers are producing even taller tires, called Group 49 and Group 50, with tire diameters of 7.2 ft. and 7.6 ft., respectively. In one roll, these tires can cover up to 22 ft. (see RCI chart below).

These extra-tall tires can be found on 4-wd tractors, grain carts, large combines and, most recently, high-hp row-crop tractors. This year, Case IH, New Holland and John Deere came out with high-hp row-crop tractors that can fit a Group 49 tire. The tires are available on other makes and models, too.

Tire makers say these larger-diameter tires are able to transfer more power to the ground and provide better traction than the next size down without increasing soil compaction as machines grow in size. This change in classes happens roughly every four to five years.

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“We are in a transition in equipment in terms of size, and we, as tire makers, have to keep up or stay ahead of it,” says Tom Rodgers, director of sales and marketing for Firestone agricultural tires. “Tractors keep getting bigger in terms of horsepower. Farmers can now get a 360-, 380-, 390-hp row-crop tractor, which 10 to 15 years ago was considered a big 4-wd. Group 49 tires give you more tire to carry the load and transfer additional horsepower at lower inflation pressures.”

Every size jump in equipment requires a corresponding jump in tire groupings, which are roughly separated by 4- to 5-in. increments, Rodgers says.

RCI, or rolling circumference index, is a way of classifying tires by size. Tires are categorized by their RCI, or rolling circumference index, a group designation that corresponds to tire size. RCI is used to select the proper size of tire for your tractor. All tires in a group have a similar rolling circumference (the distance a tire travels in one revolution) and overall diameter, or OD (also known as the height of a tire). Tires with a higher RCI cover more distance in one rotation than tires with a smaller RCI. There is roughly a 4-in. difference in diameter (height) between each RCI group. The taller the tire, the more you can reduce the air pressure to increase traction and leave a bigger footprint, according to tire companies.

When you change the size of the tire, the geometry of the machine changes, too, he adds. “As you go up in tire size, equipment manufacturers must change the height of the axle and fender to make room for the bigger tires,” he says. “The machine will sit up higher, which will affect the height of a tractor’s 3-pt. hitch, the ground-engaging components on an implement or the feeder house of a combine. So growers will notice some differences on the machine, too, when they move to a different size of tire.”

Benefits of buying tall

There are a number of benefits to running a taller tire — the biggest being load capacity, says Scott Sloan, ag products manager for Titan and Goodyear Farm Tires.

“A Group 49 tire is going to be able to better handle the load of today’s high-hp tractors better than a Group 48 tire of the same width,” Sloan says. “By being able to handle the load without raising inflation pressures, the grower can expect less ground-bearing pressure, which leads to better flotation and less soil compaction.”

Sloan says Goodyear Farm Tires was one of the first companies to offer a Group 49 tractor and combine tire starting in early 2000, and now offers Group 50 and Group 51 tires, too. “For row-crop applications, Goodyear offers Group 49 tires in section widths ranging as low as 480 (18.4 in.), as well as tires as large as Group 51 in section widths as low as 320 (12.4 in.).”

Whether a farmer should go with a larger tire size will depend on the load, application and design of the machine, tire companies advise. Older-model tractors are not designed to fit the new size categories. Currently, they are offered as an option on new-model machines.

The larger tires are available in both standard and IF, or increased flexion, versions. IF tires have more flex in the sidewalls to allow for more air and a bigger tire footprint. Farmers should work with their dealer to determine the best size and type. The bottom line for farmers is that the larger sizes will give them more options during the tire-buying process.

“For new equipment purchases, a grower may get the opportunity to specify a Group 49 tire as an upgrade versus a shorter tire,” says Sloan. “It’s important that the grower looks at the load-carrying capacity and inflation pressure benefits that come with running a taller tire. It’s also important to look ahead to the future, knowing that any replacement tires will need to have the same size of rim.”

These larger tires will cost more than the mainstay Group 48 because of the additional rubber and materials. But tire makers say the return on investment will come through better traction and less soil compaction due to the bigger footprint.

Bigger tires coming

So how much bigger will farm tires get? Last month Michelin introduced what it claims is the largest tractor tire in the world. The Michelin AxioBib IF900/65R46 is a Group 50 tire that measures 900 mm wide (2.5 ft.) and 2.32 meters tall (7.6 ft.). The tire can carry loads up to 10,600 kilos (22,000 lbs.) and run at speeds of 65 km/hr (40 mph). It is designed for 4-wd tractors rated 350-plus hp. It also fits combines and forage harvesters.

The company says this taller, larger-diameter tire provides 12% to 15% better traction than the next size down, a 15% longer footprint, and increased fuel savings due to the length of the tread, which makes movement more efficient. The tire features Michelin’s Ultraflex technology, which allows it to carry heavier loads at lower inflation pressures.

The tire was showcased on a New Holland T9 4-wd tractor at the global farm show Agritechnica. Earlier in the year, at the Farm Progress Show in Decatur, Ill., Michelin displayed a prototype of the tire in a smaller version.

Currently the tire is available only in Europe to meet road-width constraints of 3 meters (10 ft.). The U.S. also is being considered as a potential market. Further commercializing will depend on the companies who make the equipment, says Emmanuel Ladent, president of Michelin’s strategic business unit for agricultural tires. “The technology exists,” Ladent says. “It is available in our showrooms now.”

Ladent says an even bigger tire is in the works to further increase traction and load-carrying capacity. “Now, we already are working on the tire of tomorrow.”

How to read a farm tire

Here is what the numbers and letters on the side of a tire mean.

>EXAMPLE: 500/70R24 

500: width of the tread in mm

70: sidewall aspect ratio, or the ratio of sidewall height to tire width at the tread (indicating that the sidewall height is 70% of the tread width)

R: radial tire (vs. bias ply)

24: rim diameter in inches