If your driving top speed is the same as the maximum allowed speed on a specific road, the later it is possible to achieve only in theory.
Any hindrance (traffic, bends, crossings etc.) allow only a percentage of it, in my experience between 90% (motorways) downto 50% (residential roads).
I normally drive around speed limits, which are 130 (motorway), 100 (trunk), 90 (country roads, if roads and speed limits permit) and 50 (city limits), so I created my own "MyCar" profile with my adjusted speed averages(!):
Motorway 80/110
Primary Road 70/90
Major Road 60/90
Secondary Road 40/60
Local Connecting Road 40/50
Local Road of High Importance 35/45
Local Road 30/40
Small Local Road 25/40
And I always use the "Use speed limits" setting.
These values are based on experience and give me a quite correct prediction. It may be a bit pessimistic if there is no traffic, but errors are around 5% or less. In my opinion it is always better to arrive a little early than to lag behind the estimate from the Navigator.
Play with the values and find your own, there are no "best" values for all countries and all drivers.
MapFactor has the default speed setting for a car set as (Urban Speed /Extra Urban Speed):
Motorway 80/130
Primary Road 50/90
Major Road 50/85
Secondary Road 50/80
Local Connecting Road 50/70
Local Road of High Importance 50/60
Local Road 40/60
I assume you are using the free, Open Street Map version.
Motorways (in OSM) are Motorways in MapFactor Navigator, a Trunk (OSM) is a Primary Road, Primary Road (OSM) becomes a Secondary Road in Navigator,
Secondary (OSM) is a Local Connecting Road and so on, read
http://forum.mapfactor.com/discussion/comment/64/#Comment_64
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