A vehicle mounted compass needs to be swung. You take a bearing from a known objects position and find the error. You then correct for this error. The correct way is to fix magnets (aircraft version) or iron masses (old maritime solution) so the needle points the right way. Look at a ships binnacle in your local maritime museum, those 8 inch iron shot either side of the compass aren't there to hang your jacket on, they're the rough adjustment to allow for a hundred tons of steam engine a few feet away. The fine adjustment is under the dial and would usually be done at each port, hence lots of harbours have a location known as Compass point.

So called vehicle compasses are set up for an expected magnetic field (most 4x4's have 200-odd KG of diesel engine in the front) so they are better than the walkers compass in the tank bag. However, they don't entirely account for your vehicles magnetic field and i'd guess are worse on a bike where the small engine is below/behind (don't know, I never compared them). Hence, in the emergency where you know the paved road is somewhere North and the next town is west down that road, you will travel in roughly a straight line (actually a really huge arc as oposed to the 50 mile circle a compass pointing at the engine can produce). However, if the target is a 55 gallon drum of petrol in the middle of Australia, I'd want to get 10m away from the bike and do things properly every half hour or so.

This is where the sun compass comes in, if you know local solar time and have the proper scale on the dial for your latitude, you can keep moving just by keeping the shadow on the right time.

Walkers get stuck in Scotland because they need accuracy to account for their lack of range. A bike with 2 gallons of petrol will hit a road and get out so long as the rider uses his map. A walker with nothing but a Mars bar and a can of fizzy pop in their pack can't do a 50 mile detour without freezing to death, and will become more disoriented by the time it takes them to go round an obstruction. Of course a biker with a siezed engine is just a badly dressed hiker........

Andy