When I made this photo, little did I know that I would spend so much time researching steel wheel tractors with iron lugs and cleats. An excerpt from "Tractive Efficiency of the Farm Tractor": 8. On soil with a loose surface, but firm subsurface, a spade lug 9 inches long reaching firm soil resulted in a slightly increased efficiency over 6 and 7-inch lugs but was less than for 4 and 5-inch lugs. Gray County, Kansas.