I live in a rural area with a lot of state highways that my TomTom One (3rd version) insists have a 30 mph travel speed estimate, despite being 50 mph routes with next to no traffic. As a result, it tends to take me out of my way to find interstates on long trips (ie the ones I would actually need a GPS for). After some Googling I found some others also have this trouble.
If you use the "limited speed" travel option, you can slow the estimates for interstates down relative to other roads, changing the path planning to be more reasonable. Since some roads actually are slow, the naive theoretical value of 65*30/50 = 39 is too low (and may run afoul of highway 40 mph minimum speed limits on some units, I suppose). In my experiments, 45 mph seems to work well. For recurring trips, I find the best top speed for that route. In the past I'd need two or sometimes three waypoints to force it onto a reasonable path, which is a slight annoyance since it only allows one waypoint on a route so I'd have to stop to transition. Also, my unit has a pretty limited number of favorites and all those waypoints add up. With this new method, usually none are required unless a truly specific route is needed.
The travel time and arrival time estimates will be totally messed up, but they weren't very good to begin with. For recurring trips, I add the actual travel time and specific top speed to the favorite name.
Hope this trick helps some of you!