In these days of consumer "safety" it is doubtful any stock speedo would show under the actual speed. I sure have never owned any motorcycle like that.
...in which case I think the Motoclassic tach/ digital speedometer is a wonderful thing. How readable is that thing, anyhow? The digital display seems a bit tiny, and I'm worried about it becoming either a distraction while riding or a useless gadget if it's entirely impossible to read while riding.
I've got about 1500 miles on mine since installation. Perfect so far. The digital display is quite easily read. I have the green? display. The one they say is the brightest.
The display always shows the speed, which is acurate to half a mph over the speed range I've checked, up to 65 or 70 mph.
Under the speed you can select from a variety of options to monitor if you like. It defaults to trip meter.
If you were to get one, you'll need a momentary switch. They have a very nice waterproof one but will need a bracket or other mounting method supplied by you.
They also have a dual switch with a machined aluminum housing, very nice and pricy. But it looks good and you have another momentary switch for anything else you may add at a later time.