Nuvi 765T - possible to tailor typical road speeds?

I have a Nuvi 765T and was wondering whether it is possible to tailor
the typical road speeds it uses when working out routes and ETAs.
Or even just to assign preference weights to certain types of road.
I do find that it is often quite optimistic about ETAs early in the
journey, having to re-adjust them later on, particularly if the
traffic is slower than the speed limit. It appears to expect that
we will be able to drive at the speed limit all the time.

A friend of mine has a built-in satnav in his car, and was telling me
that he has lowered the predicted speed for non-motorway roads, so
that when calculating routes, it prefers motorways if possible. It
also seems a little better at predicting arrival times in advance.

Cheers
Tony
--
Tony Mountifield
Work: tony@softins.co.uk - http://www.softins.co.uk
Play: tony@mountifield.org - http://tony.mountifield.org