Look at Your car profil and add some preferences to motorway road. ... or edit map at openstreetmap.org and add speedrestriction (if You really know it) to this shorter road (edit in Potlatch is very simple).
@tvo There is link to version 1.6.xx The newest maps and program/apk are better and better.
I found the car profile where you can change road restrictions. I get the feeling you can program it so that it takes one type of road more often than another type (percentage preference). Can someone verify that?
Also, I don't know the speed restriction for individual roads. But, Google street view makes it really easy to figure that out.
Just another stupid example: Germany, Rhöndorf, Bundesstraße 42, North to South the exit just before the city centre. On the Bundesstraße 42 speed limit is 100 km. On the ramps as well (general speed limit applies) but actually it is impossible to keep 100 km the ramps down, crossing the urban road and the other ramp up.