Simplistically I assume that chronometric speedos and rev-counters convert revolutions of the cable drive to a "pulsed" movement that outputs to a graduated movement of the needle. I think later instruments use magnetic force to drag the needle round. You would have to make sure that the gearing suited the amount of movement at the needle and make up your dial marking by checking against maybe another rev-counter. Only guessing, an expert might tell us something different.How similar are mechanical speedometers and rev counters? apart from the dial face how complex is the change for a specialist and has any nonspecialist here done it?
I would have to agree with you Ed. What I have read on Smiths speedometers is the numbers on the face indicate the model and the number of rotations of the cable per mile. Which I take is for there for the odometers but it would stand to reason that if the number is say 1000 and I cover a mile in one minute the cable turned 1000 rpm. So if the cable is spinning a 1000 rpm the needle will move to the 60 MPH mark. So if I know the ratio of my speedometer drive I know how many RPM the cable is doing for a given speed. A tachometer that is driven from a gearbox on the end of a camshaft is running at 1/4 engine speed times the gearbox ratio. If that gearbox ratio is 1:1 for argument then the cable is turning at 250 RPM for an indicated 1000 RPM of engine speed. So if you were to hook it to a cable turning at 1000 RPM is would indicate 4000 RPM.Simplistically I assume that chronometric speedos and rev-counters convert revolutions of the cable drive to a "pulsed" movement that outputs to a graduated movement of the needle. I think later instruments use magnetic force to drag the needle round. You would have to make sure that the gearing suited the amount of movement at the needle and make up your dial marking by checking against maybe another rev-counter. Only guessing, an expert might tell us something different.