Tool for changing road_speed

Is there a tool which allows for changing road_speed from e.g. road_speed=7 to road_speed=6, also with large maps (about 1 GB)?
What do I want to achieve: German motorways have no general speed limit, hence road_speed=7 is used for them. But I normally drive about 110-120 km/h, that means road_speed=6 fits better. The present settings may cause large detours. When calculating the fastest route from Nürnberg to Konstanz, a route via Ulm is calculated, which is some 50 km longer than the route via Heilbronn where there are some speed limits of about 120 km/h.

In theory this can be done as speed and roadclass for each highway are kept in one byte,often found at nod2; you can change this without affecting the rest of the IMG file

You can load your IMG into GPSMapEdit, dble click on the ‘autobahn’ and change speed limit . Then save as .mp and use mkgmap to create an img

Thanks Wilpin. But that does not help: I want to change that for all motorways which have a higher speed limit - not for single part of a motorway. And GPSMapEdit cannot load such a big map as the map of Germany.

Are you sure that this will help? I think the routing is mainly determined by the road-class, while the road-speed
is mainly used for calculating the estimated time to destination.