i think if it serves a purpose then so be it, but rpm guages when you already have a stock one i think is stupid.

i wouldnt mind an oil temp guage for my gearbox... see how well the cooler actually works

A pet hate of mine, if you are concerned about the Factory tacho reading incorrect get it callibrated, thats what I did with my 1974 gauges in the coupe and they read spot on now, even compared them to the dyno and all was good!!!