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Ross Mullen of Sports Imports writes:
The 4 speed and 5 speed transmissions in roadsters drive the speedo cable by the same method. A plastic toothed gear which is color coded and comes in variations 16-23 teeth. Each tooth variation gives roughly a 10% change in speedo readings. This gear is located at the transmission end of the speedo cable and can be changed with transmission in car. It is best to drain the oil before removing the gear which is held in place by a 10 mm bolt and small tab. before removing, estimate the inaccuracy by having someone pace you and measure what your gauge reads when they are doing 50 mph. If yours reads 55 then you are out 10%( 50 x 2=100 5X 2=10%) Most 5 speeds come with 18 tooth gear normally yellow. Dropping or adding a tooth will compensate to make your a speedometer a little closer to accurate.
Increasing the tooth count on the pinion will decrease the MPH count. In other words, a 10% off 55 MPH reading with an 18 tooth pinion gear can correctly read 50 MPH by installing a 19 tooth pinion gear. Ross Mullen sports imports.
Ross is one of our vendors and has a wealth of information on his web site. You can email him at info@sportsimports.ca or visit his site here at Sport Imports-Ross Thanks for sharing Ross!