Just in case there is some one building a 1 to 1 pit.
https://www.ebay.com/itm/Electronic-Tachometer-RPM-Percent-Bendix-Corp-Dated-1974/283869846152?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908131621%26meid%3D40587d859d3046c9bfe26fdabda76aea%26pid%3D100678%26rk%3D3%26rkt%3D15%26mehot%3Dnone%26sd%3D233554734018%26itm%3D283869846152%26pmt%3D0%26noa%3D1%26pg%3D2380057&_trksid=p2380057.c100678.m3607&_trkparms=pageci%3Adb852dac-93a0-11ea-8d2b-74dbd18096af%7Cparentrq%3A047972311720a0f2cc090eb4fff24258%7Ciid%3A1