You can also buy the Kuryakyn digital speedo. You will then have the option of having it display either....the downside of course is that that little puppy is expensive! This I know from experience...I own one.
My 95 tour Classic was made for export to a dealer in Saudi Arabia and only registers in kilos .I use the 5/8ths rule. 100 kilos is 62MPH so on an american spedo 62 mph is 100 kilos. I believe on your spedo you multiply the speed by 8 and devide by 5. 50mph is 80 kilos ,40mph is 64 kilos etc. It takes a while to get used to but could sav you a lot of expense converting to another spedo.As a matter of fact, do the figures of what equals what and make a small chart and put inside your fairing just for the trip.
I just thought of the cheapest way to solve the problem. Take a good closeup pic of the spedometer in your car or truck and put it next to your spedo. All modern spedos have both kilos and mph on them . When tthe trip is over, toss it.