The other option, if you have one or have access to one, is to use GPS unit. That will give you a pretty accurate reading of your true speed. I use it so that I can set my cruise control to as close to the speedlimit tolerance level as possible :-)
The rest of the time, it ensure I don't get a speading ticket, even if I am slightly above the speedlimit. These Aussie cops have nothing else to do than raise revenue through speeding fines. Here in Melbourne the tolerance limit on urban roads is 3km/h. That's 2mph in normal speak - can you believe it!
