_P_ _L_ = -------, (3) μ + _g_ where _L_ = weight of load drawn, including weight of vehicle (subtract weight of vehicle for net load); _H_ = weight of horse; _T_ = weight of tractor; _P_ = effective tractive force exerted (available engine effort); μ = coefficient of road resistance; _g_ = grade (gradient) = tangent of angle of incline, nearly the same for small angles as the sine of the angle of incline, that is, the height of the incline divided by its length; _t_ = the direct pull of the horse divided by the weight of the horse; _h_ = horse-power = work of 33,000 ft.-lb. per minute. _v_ = velocity in miles per hour. Equation (3) indicates that the load, including its own weight, that a truck or an automobile can draw varies directly as the horse-power exerted effectively, and inversely as the velocity. Also it decreases as the coefficient of road resistance, μ, and the gradient _g_ increases. The resistance coefficient, μ may include axle or internal resistance of the vehicle plus road surface resistance plus air resistance. The axle resistance is nearly a constant, the road resistance likewise, but the air resistance depends upon the speed _v_, varying approximately as the square of the velocity. W. S. James, in the _Journal of the Society of Automotive Engineers_, June, 1921, uses the formula _F_ = _CAV_² where _F_ = the wind force in pounds; _C_ = a constant, varies from .003 to .004; _A_ = frontal area of automobile in square feet approximately 26; _V_ = velocity in miles per hour. His researches show that the available engine effort _P_ of equation (3) or horse power _h_ is not quite constant but varies with the speed. His table follows: ---------+-----------------------+--------------------------- |Available Engine Effort| Car Speed| Per 1000 lb. of Car |Air Resistance Per 1000 lb. m.p.h. | Weight, Lbs. | of Car Weight, Lbs. ---------+-----------------------+--------------------------- 15 | 107.3 | 4.9 16 | 105.2 | 6.8 20 | 107.6 | 8.8 25 | 106.0 | 13.4 30 | 103.9 | 19.2 35 | 101.2 | 26.0 40 | 98.0 | 34.1 45 | 94.1 | 43.4 50 | 86.8 | 53.8 ---------+-----------------------+---------------------------