I had one of those as kid- in fact several of my friends, andliked it then. I don't know about at faster speed, but it's a small investment.
I've been wanting to get one to try, but not a priority and I'm a victim of CREDIT COUNSELING!
I've put a 27 inch wheel on the front, and the speedo claims to be for both 26 and 27 inch wheels, which isn't really that possible- the 27 will roll further per revolution.
So I wonder if it's more accurate for one size than another? They seem to be just like the ones of 50 years back- when 27 inch was not around- so I wonder if they just say they work for 27 to selll more of them.
I guess I could have someone check a speed against a car speedo, and then figure in any rate of variance.
heres a link to one:
Speedometers - Dial Speedometer 26"/27" MPH or KMPH | BikeWorldUSA | Bicycle Parts and Bike Accessories