Jim-
I have a tach on one of my cars. It was installed in the car before I bought it, so I left it. On the other car, I temporarily hooked up my "tune up" dwell/tach to the coil and went for a drive. If you note the RPM on the tach in each gear at some specific speed (bring a passenger to do this and write it down) you can easily use the speedometer in the future as a tach.
You don't need a tach installed all the time. The tire size and gear ratios do not change.
The RPM in 2nd gear at 20 MPH will always be that, and you can scale up and down from that to any speed/RPM (the RPM at 10 MPH in 2nd will be half of that, at 30 MPH will be 1.5 times that, etc.).
The RPM in 3rd gear at 30 MPH lets you scale up and down from that to any other speed/RPM in that gear.
