Through the years, speedometers, more-or-less, have been accurate. The emphasis here is on "more-or-less". Our article on a 1955 Chevy Road Test indicates the accuracy of its speedometer:
Indicated 30 mph...........28.3 mph actual|
Indicated 40 mph...........37.7 mph actual
Indicated 50 mph...........47 mph actual
Indicated 60 mph...........56.4 mph actual
Indicated 70 mph...........65.8 mph actual
Indicated 80 mph...........75.3 mph actual
Indicated 90 mph...........83.4 mph actual
That's an error of 6.6 mph at 90. Fortunately, actual speeds were lower than that shown on the speedometer. At least with this setup, your speedometer didn't cause you to get ticketed.
If a speedometer does not seem to be indicating the correct speed it can be checked by the following method:
Check the odometer on a measured mile. (You've most likely seen measured miles signs on freeways.) Choose a speed at which you wish to test the speedometer and drive the car at exactly that speedometer speed for one mile, at the same time recording the time with a stop watch. Use tenths of a mile portion of the odometer to measure a mile. The odometer can be read most accurately just as a number reaches the top of the odometer opening. It is advisable to make the test at several different speeds, recording the speed shown on the speedometer and the number of seconds required each time to cover the mile.
To compute the actual speed of the car, divide 3,600 by the number of seconds required to traverse the mile. For example: if, while traveling at 70 mph according to the speedometer, the time required to travel one mile is 50 seconds, the actual speed is 3,600/50 sec. or 72 mph. If the car is driven at 60 mph according to the speedometer and it takes 60 seconds to traverse a mile, the actual speed would be 3,600/60 sec. or 60 mph, showing the speedometer to be exactly right.