TACHOMETER AND SPEEDOMETER ACCURACY                                       by Ben Prince      Pure-of-heart racers will tell you that the speedometer is an unnecessary instrument.  To them, the tachometer is the key gadget for all-out driving, providing the necessary information for maximum performance.  For many of us lesser souls, both gauges are nice to have.  The tach is indeed important for engine monitoring, and the speedometer displays speed limit transgressions, quickly indicating the risk of a night in slammer!        Unfortunately, the tach and speedometer may not be particularly accurate.  There are errors associated with linkage, calibration and internal wear that may accumulate over time, frequently compounded by needle oscillations.  To find out how good or bad accuracy may be, here are some simple tests. Relative Comparison -------------------                One quick check is to compare the readings from the two gauges.  Once a gear is selected, there is a particular RPM for each number on the speedometer.  The following lists these correspondences when the car is in fourth gear at 60 mph:                  Vehicle        Speed        Rear end ratio      Engine RPM      -------        -----        --------------      ----------      TC             60 mph            5.125             3787      TD/TF          60 mph            5.125             4167      TD/TF          60 mph            4.875             3963      TD/TF          60 mph            4.555             3703      TD/TF          60 mph            4.3               3496      For the TD/TF figures, it is assumed that Michelin 165 x 15 tires are used, rotating 813 times per mile (in accordance with Dr. Carl Cederstrand's writings).  For the TC with nineteen inch wheels, the RPM above is based on 739 turns per mile, a derivation from an original TC advertisement.      If your car has tires or wheels that are somewhat unusual in size, the table above may not apply.  It is nevertheless easy to compute the RPM at 60 mph (in fourth gear).  After checking tire inflation, roll the vehicle on level ground in a straight line until the rear wheels make twelve complete revolutions.  Then measure the distance transversed.  To determine what the RPM should be at 60 mph, multiply 63360 times the rear-end ratio and divide by the measured distance.  Suppose, for example, the distance is 79.5 feet and the rear-end ratio for your car is 5.125.  Then 63360 x 5.125/ 79.5 = 4085 RPM.      To conduct the test comparing the tach and speedometer, merely drive the car at 60 mph in fourth gear and see how closely the RPM matches the appropriate figure in the table above.  It is important that fourth gear be selected since it typically is a 1:1 ratio, allowing the test to be valid regardless of gearbox -- whether it is an MG transmission or one of the conversions to a Datsun, Ford Sierra or Toyota gearbox.                        Although unlikely, it is possible for both instruments to be in error in the above comparative test, but yet show "agreement".   They could be accurate relatively speaking, but not in an absolute sense.  The way to eliminate this circumstance is to test the gauges separately. The Tachometer --------------      It is easy to check the T-series tach with an electronic RPM/dwell meter.  (With my meter, one wire goes to the primary terminal on the distributor and the other to ground.)  Merely compare the RPM reading on this gauge with the vehicle tach at various engine speeds.  These observations will hopefully yield reasonable agreement, but don't expect too much.  From talking to others and based on my own experience, consider yourself lucky if the tach is within 200 RPM at cruising speeds.  And more often than not, the tach will read on the high side.                      It is perhaps amazing that the tach works as well as it does when one considers the convoluted linkage.  The crank turns the generator via the fan belt, each crank revolution resulting in 1.158 turns of the generator.  At the rear end of the generator is an adapter that connects to the tach cable.  This adapter reduces the speed of the tach cable by .4444.  And then inside the tachometer, the read out is doubled.  Combining all these numbers results in an overall ratio of 1.03.  (Note 1.158 X .4444 X 2 = 1.03.)                  This final number is interesting, showing that there is a built-in 3% linkage error.  For example, suppose for a moment that this linkage error is the only error and the engine is turning 3500 RPM.  Multiplying by 1.03 results in 3605 being presented on the tach, a figure that is 105 RPM high.  On the other hand, this 3% error is perhaps of little practical significance, since the tach needle often jiggles over 100 RPM.   It is nevertheless interesting to note that the tach will probably read on the high side, even when it is freshly overhauled.  (To help, you might have the instrument shop bias the tach slightly downward to compensate for the 3% pulley/gearing error.)       The Speedometer ---------------      After checking tire pressures, an easy speedometer test is to drive a measured mile, or better yet ten miles, if you can find a stretch of highway with the desired markers.  Try to hold the speed at 60 mph.  The odometer should reflect the actual distance, and the elapsed time should be one minute for each mile (recall that 60 mph equals a mile a minute).                     Another approach for testing the speedometer is to follow another car and compare the gauge readings.  Try to follow a modern vehicle that has cruise control.  (Assuming tire size and pressures are correct, the modern car's speedometer should be a fairly accurate benchmark.)  To coordinate the exercise, it is desirable to be able to communicate between the cars, cell phones or radios helping enormously.  The modern lead vehicle should proceed at selected speeds with cruise control engaged.  Any speedometer errors may then be noted for the T-series chase car.
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