Fastest route sometimes depends on whether the GPS is getting current traffic info and figuring it into the calculations. Garmin is also pretty good at adapting the routing to your driving habits - if you drive the same route a few times, Garmin will give that route preference sometimes. Try clearing your travel history. Double-check your routing preferences, and turn off the display option that shows all those Starbucks logos. Reset the unit. We have two units - one for the toad, and one for the motorhome. We've had almost no problem with routing. There was one instance where Garmin thought there was a length limit on a particular stretch of road and refused to route the motorhome over it . . .but I reduced the length in the RV profile by 1 foot and then it worked.
In the motorhome, I ALWAYS proof the route on the laptop to make sure it goes where I want it to. In the toad, we tend to go where Garmin wants, because we can always get the toad out of a funny situation.
al