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Unlike speedometers, tachometers don't have a drive unit to wear out. After restoring and relettering the face to 0-8,000, the tachometer is electronically recalibrated. In many cases, the original circuit board has gone bad, so Auto Instruments replaces it with a new one. Gentry says that many original tachs are off by as much as 1,000 rpm, which can be a big deal if you think you're shifting at 6,000 rpm but it's actually 7,000 rpm.