Many years ago I ran a bicycle speedo. After a while I knew what revs in which gear were what speed.

M Max ,the old Japanese after market parts company, used to sell a speedo to use with their M Max mc28 card. I had one, along with their card fitted to an old bike of mine. It was about an inch high and 3” wide and stuck on just above the clocks. Worked a treat and also did 0-60 times