My 27 has a speedometer, but I haven't hooked it up in the 25 years I have had the car on the road. What I did was I went for a ride with a hand held GPS, and marked the tach at 35, 45, and 55 mph. I put a little painted line at those speeds on the face of it. So when I am in high gear I can look at the tach and know how fast I am going.
But when I put the 4.30 gears in it I had to redo it but it worked out perfectly because every 1000 rpms is 20 mph, so if I am taching 2000 I am going 40 mph and 3000 is 60. I just have to do a little math in my head to figure out things like 35 and 45, but it is easy.
Don