Is your alternator belt a V or ribbed? How does the alternator belt fit the pulley? A V-belt settles at a different radius from the axle depending on the width of the belt and the width of the pulley's groove. As this radius changes due to belt wear or just differences between manufacturers, the ratio of crank shaft revolutions to alternator speed will change and throw off your tachometer. Your harmonic balancer (pulley on the crankshaft) is probably a larger diameter than the alternator pulley so while this effective belt diameter changes with belt thickness as well, it is at a different %.
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