When I go racing, the Autometer Monster tach vs. the stock tach reading can vary as much as 600 rpm on my car after 5000. Also, at idle, every now and then I'll show a 50-350 rpm difference, though that seldom happens.
Had an Autometer tach in for awhile. Didnt need it. Sold it. Factory tach in my car was right on or within 100 rpm. I drag race a ton, and really didnt need the extra tach. I dont use a shift light as I vary my shift points in the higher gears.
My best friend is an electronic engineer and actually designs tachs where he works. Most tachs are +/- 3%. Or no more than 1 pointer width off (about 100 rpms). Its less accurate at low and high RPM's. I had several of his tachs in my car and my stock unit is fairly accurate. I do suggest a good tach if you drag race alot.
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