Curiosity proved confounding when Leonard Bashford, director of the University of Nebraska Tractor Test Laboratory, decided to test narrow radial tires against wider units and compare their performance.

"We looked only at tractive performance," Bashford says. "Logic tells you that the wider tire should perform better. You've got more lug in the ground for more bite and more pull."

But the test data show that, on both untilled and tilled wheat stubble, the narrow tires out-performed the wider units in both tractive efficiency (TE) and dynamic traction ratio (DTR). TE is a measurement of how efficiently a tire converts axle horsepower to drawbar horsepower. DTR is the pull/weight ratio that measures how much weight a tractor can pull compared to its weight.

The narrow tires in the tests were 18.4R46s and the wide tires were 710/70R38s (approximately 28"). Both sets of tires were mounted as duals on a mechanical front-wheel-drive tractor with inflation pressures of 12 psi and 6 psi, respectively.

A wheat stubble field (Sharpsburg silty loam) was divided in half for the tests. Half the field was standing stubble, the other half was tilled to 9" with a sweep plow, leaving residue on the surface.

"At 10-15% slip, a normal operating range for farmers, the narrow tires showed a very observable advantage, especially from a TE standpoint," Bashford says. "We didn't expect to see the performance differences between the two tires favor the narrower tire."

Field conditions may explain some of the results, says Ken Brodbeck, project engineer for Firestone Ag Tire Corp. "On a firm soil surface you won't see as much difference in performance between tires. As soils become more moist, or mellow, you can expect wider tires to perform better.

"If you farm with several tractors, it makes sense to equip them with different size tires, 18.4s for row crops and 20.8s and larger for primary tillage work," Brodbeck says. "The 710s run at half the tire pressure of the 18.4R46s and the wider, lower-pressure tire is going to work better in loose, wetter soils and cause less compaction. Tractors over 200 pto horsepower can have a difficult time controlling power hop unless they use larger tires."

If power hop and soil compaction are not a problem, and you only farm with one large tractor, the 18.4s can handle both your row crop and minimum tillage needs, Brodbeck says.