Will an older guage fit on my bike?

I have an 87 Honda Shadow VT700 that needs its speedometer replaced. Will one from 83-85 fit?

Likely. Most Japanese speedometers are cable driven from the front wheel hub with the exception of some of the newest computer sensored from magnetic pickup units. That vintage interchange is more than likely to fit on cable- but might need a bit of mounting bracket adjustment, maybe a bit of indicator light rewiring- but main speedometer function of keeping speed below police ticket level should work. I'm running odd Honda speedo on my little CM400 with Yamaha front wheel- had to do a minor cable end adjustment but it works good enough for students to stay legal.

From 84'-87' the VT700 Shadows changed minimally and all the changes were simple cosmetic things, so anything in that range made for the 700 should work just fine. The 83' model was a 750, so not sure if it would work or not. So just make sure it's for the 700, since Honda did make the 750 in the same years for sale outside the USA(in Canada and Europe) and they also had the VT1100.