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Tire sizing 101: What’s your RCI?

Tire sizing 101: What’s your RCI?

Farm tires are getting larger to support the weight of combines, tractors, and wagons, which are growing in size.  Folks in the tire industry use a Rolling Circumference Index (RCI) chart like to categorize farm tires. Tom Rodgers, marketing manager for Bridgestone-Firestone, provided us with this RCI Chart to translate the language of tire sizing.

Rodgers says that up until 2009 the largest size category on the replacement tire market was “Group 48,” defined by a rolling circumference of 243 in. and overall diameter of 81. In (think 6’8” tall). This size of tire has gained significant popularity because of the increased capacity compared to same section width tire from RCI 47.  The RCI 48 tires also provide a larger footprint for better traction and less compaction.

The next size up is RCI 49 tires, which are just starting to be introduced by the tire industry.  The 710/75R42 is already in use in Europe as singles on Row Crop tractors.  For 2009 Firestone will introduce two new sizes in RCI 49.  The first: an IF480/95R50 with AD2 Technology, which is the big brother to the 480/80R50.  The second is what will be considered the next generation combine tire – a 900/75R32 (36” wide and 85” Tall). 

“This new RCI 49 Combine size will be a great match for the next generation of combine’s introduced by manufactures,” Rodgers says. 

He says larger-capacity Class 9 and 10 combines--with larger grain tanks, wider corn soybean heads, as well as larger Tier 4 engines--will push the limits of the industry’s current tire offering.  “The 900/75R32 will operate at lower pressures, for the same load, than any other single drive tire on the market today,” Rodgers says.

For other tire trends, check out the December 2010 issue of Farm Industry News

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