MILES PER GALLON

Southland Times, Issue 20950, 6 December 1929, Page 9

MILES PER GALLON

CONTROLLING FACTORS. SOME EFFECTIVE TESTS. “It all depends on the driver,” stated a General Motors official, in discussing how many miles a gallon of benzine will drive a motor-car. The most important factor in benzine mileage lies outside the control of the engineers who design the car, the manufacturer who builds it or the service man who maintains it. That factor is the human element as represented by the individual car driver. The question of benzine mileage is considered of such importance by many motorists that much time and effort has been spent by automotive engineers in studying benzine efficiency. All sort of tests have been made to get the facts. While many of these are technical in character and require the use of special engineering instruments, the one most convincing to all was the simplest. It required no special instruments and is one that easily can be duplicated. Two motor-cars of the same make, carefully tuned to deliver exactly the same benzine mileage at the same speed under the same driving conditions were run over an eighteen-mile stretch of comparatively level road in exactlv the same elapsed time. AVhile both cars started side by side and ended the run at exactly the same time, they were' driven under widely varying conditions to definitely prove that human element and driving habits are the governing factors in benzine mileage. Car Number 1 was driven at a variable speed. It stopped for 10 seconds every four-tenths of a mile, shifted from first to second gear, speeded up in second gear to 35 or 40 miles per hour, then shifted into high gear and was driven as fast as necessary to pass Car Number 2. Then this process was repeated again and again over the 18-mile course. At the end of the test car number 1, driven at variable speeds, had consumed nearly twice as much benzine as had car number 2, driven at a constant speed. The test mileage for car number 2, driven at a constant speed of 30 miles per hour, was 19.7 miles per gallon, while car number 1, that hopped back and forth from a standstill to 60 miles per hour, had a mileage of but 10.4 miles per gallon—a difference of 9.3 miles per gallon or nearly 90 per cent.

The difference was entirely due to the different kind of driving, and is proof of the fact that if a driver insists upon staying "out in front”—if he insists upon running his car to a fairly high speed in second gear—he must expect less benzine economy. It also shows that the driver who handles his car in a moderate manner, getting under motion in first and second gear, then striking into high, will operate his car much more economically than will the flashy type of driver. The fewer the stops and starts, the greater the economy. Plain Speed Tests. Driving speed, whether constant or vanable, is another important factor governing benzine economy and over which the car driver alone has control. Recently tests were conducted to determine fuel mileage of sixteen cars. The average for all cars which were tested, including various types of many well-known makes, was 22.16 miles per gallon at a car speed of 15 miles per hour. At a car speed of 25 miles per hour, this average mileage per gallon of benzine decreased to 20.36. When the car speed was raised to 35 miles per hour the benzine mileage was still lower, 18.24 miles per gallon; and at a speed of 45 miles per hour, the benzine mileage was 15.51. These were the averages for sixteen cars tested, although some cars did considerably better than the average, while others fell below the average. The many factors that govern mileage are complex. Car condition, wind velocity, carburettor adjustment, grade of fuel, temperature, humidity, altitude, type of road, and topography are all faettors which have an important bearing on fuel efficiency. The average car ow-ner has little conception of the tremendous physical forces that he brings into action when he steps on the throttle of his car, and has a still more inadequate conception of the manner in which those forces govern the “miles ;>er gallon” he derives from his car. The element of wind resistance, alone, serves as a sterling example of this lack of knowledge on the part of the average owner of what governs fuel mileage. As car speeds increase, the horse pow-er required to pull any car against wind resistance becomes directly proportional to the frontal area of that car. Wind resistances, which materially cut benzine mileage, play a more important part in the speed range above 30 miles per hour than below that speed. Tests of Resistance. The factor of wind resistance is dependent upon the frontal area of a motor-car. To illustrate this point, two cars were placed on test —a light car weighing 2757 pounds with a frontal area of 24.9 square feet, and a heavy car having a weight of 4540 pounds and frontal area of 29.6 square feet. The heavier car weighed 65 per cent, more than the light car though its frontal area was only 16 per cent greater than that of the light car. At 20 miles per hour the light car gave 24.6 miles to the gallon and the heavy car 13.4 miles, or an advantage of 83 per cent, for the light car. At 40 miles per hour the light car gave 20.4 miles to the gallon, and the heavy car 12.8 miles, or 59' per cent, in favour of the light car. At 60 miles, the light car showed 9.8 miles to the gallon, compared to the 8:28 miles for the heavy car, or only 19 per cent, advantage, while at 70 miles the light car gave but 7.2 miles to the gallon and the heavy car showed 6.2 miles, which brought the advantage of the light car down to 16 per cent.

The relative effectiveness of wind resistance at 20 miles per hour and 70 miles per hour between the light and heavy car is indicated by the fact that while the light car at 20 miles per hour has a benzine mileage 83 per cent greater than the heavy car, that advantage is cut down at 70 miles per hour to but 16 per cent. The foregoing test of the relative effectiveness of wind resistance on a light car or heavy car materially contradicts widelyaccepted theories, and is conclusive evidence of the error of attempting to discuss fuel efficiency with any degree of authority when not substantiated with technical data with which to prove the claim. It seems needless to point out the folly of trying to define any specific number of miles per gallon which can be expected from driving any make of car. The fact bears out the statement. “It all depends on how you drive.” The driver who insists on keeping in front of other cars all of the time is the one who gets the least benzine economy. Benzine economy is the sacrifice price that must be paid for high speed or brilliant performance.