Interesting. I normally mentally allow 4-5 hours to do it, breaks and traffic dependent. It's quite long to do none stop. Besides, who can't stop at House of Bruar for cake?! That's based on a lot of experience going backwards and forwards to Beauly and points in Glen Affric.

I think three is pretty unrealistic actually - unless you are doing a night or late evening run, it's very rare to be able to maintain an average speed close to the speed limit - the road is just too busy with things going 45-50