Need help from the big boys. More familiar to the engine driveline forum but I am facing a deadend.

I want to install a mechanical tachometer on a 261 to make sure I see what revs I am pulling. Most electronic tachs have a scale of 7 or 8000rpm........ 4000 rpm mechanical tachometer are readily available on EPay.... but no drives are ever available.

So my question is how did GM set up the old fashion mechanical tach drives??? the ones I have seen on 361 Chrysler were gear driven from the dizzy. What should or where should I be looking..??

Or..... does anyone know of a low rpm tach that is electricaly driven.

Engine is in a 1940 Chev Canadian Military Pattern 4x4 right hand drive 15 cwt (3/4 ton) truck..... top speed 45 mph with a 216... hoping to top 50 mph with a 261..... but really need to see what is happening under the hood at different speeds. Axles are 6.39 on 16 inch rims. I have used the tire diameter formula to estimate road/engine speed but want to monitor while driving.

Thanks for your help.

Bob Carriere
Ottawa, Ontario

Bob C.
CMP Heaven
The Hammond Barn