you know, the other day, I was in a CNG cab, chatting with the
cabby
the cabby says with CNG, he clocks about 340km on a full tank of
gas, which costs $20
plus, he gets a rebate off his road taxes, previously, with
diesel, he was paying $3000 for 6 months, now he pays $600 for the
same period of time ....
idea neh ...
i'm thinking of this too... considering in pure $$ i get about
6.6km per dollar.. he gets 17km per dollar...
suddenly the fugly CNG Toyota Wish becomes pretty cool 
the Prius gets 23.5km per litre still goes to be around slightly
more than 10km per dollar...