Based on my experience with Garmin's mapping data, using posted speed limits in calculating a route is WORTHLESS. For example, here on Long Island, the posted speed limits for Jericho Turnpike and the Long Island Expressway is 55mph. In reality, if you're not doing AT LEAST 70 mph on the Expressway, you're going to get run off the road or passed and cut off more times than you'd care to think about.
On the other hand, with all of the traffic lights and busy intersections, you'll be LUCKY to average 30 mph on Jericho Turnpike. But Garmin treats both roads identically and makes some of the DUMBEST routing decisions I've ever seen. I got better results with the previous generation of Garmin GPS units when I was able to input my OWN values for average speeds on freeways, secondary roads and surface streets.
I'll NEVER forget when, on my first trip to Daytona Beach using my brand new Garmin unit, it took me off of I-95 (posted speed limit 55, actually doing 70-75) and onto a secondary road outside of Washington DC (posted speed limit 55, actually NOT MOVING AT ALL because of heavy traffic congestion) all because the route would've saved TWO MILES.
The Google Navigator (which incorporates data from WAZE which Google owns) is more accurate as it gauges the speed traffic is ACTUALLY moving. That changes from hour to hour and is not static.