score:2
I would not rely on speed data in any GPS unit as a means of ensuring you are travelling the correct speed. Often-times I have experienced that they are not correct, especially with my Garmin unit. You will find that roads are well marked with speed limit signs. General speed limits can be looked up a head of time, so you can get a general idea based on type of road.
Speed data is sometimes available but limited within OSM. See here for more information on speed in OSM, including some discussions related to speed data. Note that some of this may be able to be transferred to your Garmin. See here for information about customizing maps and putting them on your Garmin.
Best regards.