In Canada, federal labor law, which most drivers/companies are operating under, are supposed to get OT after 60 hours/week, regardless if it's mileage pay or hourly pay. and it doesn't matter if the hours are accrued by time that is deemed unpaid or 'included' in your mileage pay. Most companies don't pay it because most drivers either don't know the laws or don't exercise that law but it's there and in place to be used. As well, any and all salaried jobs are to be based on 40 hours per week, anything over that either has to be paid as OT or be given time off in lieu of time worked beyond 40.
I know your guys labor laws for truck drivers sucks and I wish someone would lobby the feds to change the laws.