EFFICIENCY OF GAS TRACTORS 107 



indicated horsepower, though often lower. In the Winnipeg 

 motor contests few gas tractors have equalled their rated power 

 in brake tests. In 1910 the class as a whole averaged 86.5 

 per cent, of the brake rating on a maximum test, and 80.9 

 per cent, on an economy test. The actual drawbar or tractive 

 power of tractors cannot so easily be compared on account of 

 lack of uniformity in rating. The sales rating is sometimes 

 based on brake and sometimes on tractive horsepower, 

 while in many cases it bears little relation to either. As a 

 general rule the tractive ratings are about one half the brake 

 rating. 



The tractive efficiency of the average round- wheel gas tractor 

 on good footing ranges from 50 to 60 per cent., dropping some- 

 times to as low as 30 per cent, and occasionally reaching 70 

 per cent. In the trials at Winnipeg in 1909 seven single-cylinder 

 gasoline tractors, using comparatively low, wide wheels, weighed 

 299 pounds per inch in width of driver and 535 pounds per 

 brake-horsepower developed in an economy test. In plowing, 

 they delivered 61.4 per cent, of the brake-horsepower at the 

 drawbar and in hauling, 50.8 per cent, assuming, of course, 

 that a comparison of separate brake and traction tests is reliable. 

 Basing the tractive efficiency on the fuel consumed per unit 

 of work in the various tests, it fell to 44.1 per cent, in plowing 

 and 35.5 per cent, in hauling. Ordinarily, of course, the trac- 

 tive efficiency in hauling would be the greater, as road surfaces 

 would be more solid than that of plowing fields. The hauling 

 course for the tests afforded poor footing, as has been stated 

 already. 



In the same tests, several multiple-cylinder gas tractors, 

 equipped with high, rather narrow wheels, weighed 407 pounds 

 gross per inch in width of driver, and 416 pounds per economy 

 brake-horsepower. They were less affected by the adverse 

 conditions in the hauling tests, both horsepower and fuel 

 consumption indicating a tractive efficiency of a trifle over 

 50 per cent. The actual figures were 53.1 per cent, in 



