Aaron says: "I suggest getting the diameter of the rear wheels by
            measuring the radius perpindicularly from the ground up
            to to centre of the wheel, as this takes into account
            the 'squash' of the tyre (and resultant decreased wheel
            diameter) caused by the weight of the van."

I believe it's the circumference that is important.  Assuming that the tire 
is steel belted, it think that the circumference will be quite stable at 
any speed or any amount of squash.  Too help understand why squash makes no 
difference in circumference, visualize a flexible hula hoop.  Think of a 
mark on it and how far it rolls in one revolution.  Now visualize the same 
hoop squashed down to look more like a Caterpillar tractor track.  I think 
that you can see that one revolution will still go the same distance. 
 Indeed, if you were to cut the hoop and unroll it out flat, its length, or 
what used to be the circumference, would be exactly the distance covered 
when it was rolled one revolution.

So, having said all that, it would be best to measure the diameter of the 
tire horizontally to calculate the circumference.  For best accuracy, 
perhaps it would be best to jack the tire off the ground and measure the 
circumference directly.  Or, how about marking the tire and measuring the 
distance to roll the bus one revolution of the tire?

Thanks for listening,
Mike