Most new cars come with tachometers as well as speedometers in 2010. This makes it easy for the average driver to determine what road speeds a given engine speed will provide in different gears as a matter of curiosity. Builders of performance cars, on the other hand, must determine what gear ratios and tire sizes they must utilise to provide maximum engine power at the road speeds where they will be competing. Calculating these speeds is therefore contingent on a knowledge of an engine's power curve, tire sizes and gear ratios.
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Define the road speed application. In this example, race car owners wants to build a road car that can run rallies at about 85-miles per hour in top gear. Their engine produces its maximum horsepower at 6200 rpm (revolutions per minute) and it has a red-line of 7000 rpm. The diameter of the tires they would like to use is 24 inches. The transmission is a close-ratio four speed with first through fourth gear ratios of 2.20, 1.75, 1.3 and 1.0 to 1. With this information you can calculate the road speeds at maximum power in each of the gears, with the optimal rear axle ratio.
Calculate the distance travelled by one turn of the drive wheels in feet by multiplying the tire diameter by pi and dividing by 12-inches. Substituting the 24-inch value, 24 X 3.1416 = 75.40 inches or 6.283 feet, which is the circumference of the wheel.
Calculate the optimal rear axle ratio. If the owners want their engine to deliver peak power at 85 mph in top gear, then the optimal ratio = 85-miles per hour X 5280-feet per mile/60-minutes/hour = 7480 feet per minute/6.283 feet per wheel revolution = 1190.51 revolutions per minute. If engine speed at peak power is 6200 rpm, then a drive axle ratio of 6200/1190.51 = 5.21 to 1 optimal axle ratio.
Calculate the road speeds in each of the gears including top gear at the engine's peak and red-line speeds of 6200 rpm and 7000 rpm respectively.
First gear would provide a speed of 85 mph/2.2 ratio = 38.63 mph at peak power and redline speed would be 38.63 X 7000/6200 or 43.62 mph.
Second gear would provide 85/1.75 ratio = 48.57 mph at peak and 48.57 X 7000/6200 = 54.84 mph at engine redline.
Third gear would provide 85/1.3 = 65.38 mph and 73.82 mph respectively, and
Fourth gear would top out at 95.97 mph, with its peak power speed being fixed at 85 mph.
Tips and warnings
- Performance cars are a compromise of speed and acceleration with the pivotal point being the production of peak power being at the average speed the car is raced.
- All local traffic laws should be followed when driving these kinds of performance machines on the streets.
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