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MG MGB Technical - Speedometer & Tach correction

I own a mg td fitted with a 1964 MGB 3-main engine and transmission The speedometer and tachometer are MG TD and because the gear ratios are not the same as the MGB, the instruments do not show correct readings. Does anyone know a way to match the TD instruments with the MGB engine and transmission?
Frank Grimaldi

Frank - The speedo is easy. Just drive a measured mile (I usually do 5 miles) at a steady speed and either time it or use a GPS to calculate your true speed vs. the indicated speed on the speedo, take these figures to a local speedo shop and they will make you a ratio box to put between the transmission and the speedo. The tach is a bit different. If there is a take off for a mechanical tach at the back of the engine (cam shaft), I believe that you can couple directly to that with a cable from a MGA or early MGB. The MGTD tach takes an input that is 1/2 the engine speed, which is the speed of the cam shaft. The other alternative is to use a standard TD generator with a tach take off on the back of it coupled to a tach drive gear box. this is complicated by the fact that the generator pulley and the tach drive gear box have to give an output speed that s half of actual engine speed. Cheers - Dave
David DuBois

This thread was discussed on 27/01/2009

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