The results are affected by pre-stocking at the end of last year in reaction to the excise duty hike.
They also pay no vehicle excise duty, have cheaper insurance premiums, are exempt from London's Congestion Charge and can be charged for free at some public car parks.
The rate of taxation or rate of customs and excise duty is of great importance.
The two main current sorts of tax-fuel duty and vehicle excise duty (VED) -do not do that well.