I noticed an article in the motoring section of the County Press dated 8^th February 2008 concerning the Isle of Wight Council car parks, it said if your car falls into the Government rated bands A and B (less than 120g of CO2 per KM travelled) then you qualify for free parking in Long stay, Short stay or Shoppers car parks.
I checked with the Council (01983 821000) & it is correct!
So if you have such a vehicle you can stay in :-
Long stay for up to 8 hours
Short stay up to 3 hours +
Shoppers car park up to 5 hours
(This is the same concessions as those who pay for an Island Residents permit).

Hope this was useful!

Simon