Google "tiny tach".

It's an electronic device with a pressure transducer that clamps on an injector line and translates that into a tach reading...


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Tiny tach, later model alternator with tach wire, some gauge companies make a tach that has magnets to glue to the crank or if you don't need your vacuum pump you can get a gas distributor and use it and your current vacuum pump drive for a tach. Factory Gas tach Now Working! - Diesel Place : Chevrolet and GMC Diesel Truck Forums

With my 83 I get in the high teens for mileage, but it is mountainous and 6500 ft where I live . I also run 33" tires and 4.10's. I had 28" tires on it for a bit and the only difference was I didn't loose as much speed on some of the bigger inclines. Mileage stayed the same.