I had this issue and it completely screwed the routing on my NAV. I have a later BM54 with dynamic routing which uses journey times + traffic to calculate the best route. When the arrival times are wrong, the whole thing is pretty useless.

I don't think it is down to the wheel sizes. When mine was faulty it would basically calculate the whole journey time based on your current speed, with no consideration for other road speeds on your journey. For example, if I had a journey from Leeds to London, and started on an A or B road, the journey time would be 8 hours (double the actual time). When you get on the motorway and are lucky enough to reach a decent speed the journey time will drop to about 3 hours. Very frustrating.

I took it into the dealer and they just recoded it for free and the problem went away. My journey times are now accurate to a few minutes. They also apply the latest firmware upgrades for free.