Hi Stuart
When I blew the picture up the tread in the drivers side tire becomes visible, and appears to be as Lang said a standard Highway tire. This is not uncommon even during the war. Take a look at some pictures of CMPs taken in British Columbia during the war
http://canadianmilitarypattern.com/C...20Columbia.htm you will see that many of the trucks have regular truck tires. The picture below shows regular tires being used in less than ideal circumstances with predictable results.
One of the reasons for this was that the military tires use a lot more rubber and thus trucks operating in non-tactical areas may have been fitted with regular tires as a rubber conservation measure. Also as Lang points out regular tires may actually wear better when operating on hard roads or surfaces.
Cheers Phil