Tim, the hole measures about 2-3/4" and the closest tach I can find is 2-5/8". I got one at SpeedHut.com where you can "design your own gauges" by specifying face color, text style, needle style etc. It's not here yet. Been two weeks in the "art dept". I visited my acd see it has moved to "production".
In keeping with the sixties push truck theme, this is what I came up with -
SpeedHut Tach face 
