In message , dave F
writes
I still believe large engine capacity engines and above average car length
vehicles should face an extra tax.

Some over-egging going on here it seems:

"Larger-engined cars produce up to five times more carbon dioxide per
mile than the most fuel-efficient."

But with a free choice of high and low emissions, the best they can do
is:

"The annual duty on a Land Rover Discovery V8, one of the largest
four-wheel-drives, which emits 397 grams of carbon dioxide per kilometre
is £160 - only £75 more than for a diesel Renault Clio that emits 110g."

Which is only 3.6x

Meanwhile, I used to have two vehicles: a 700cc Matiz and a 3,000cc
Diesel Mercedes. They both did around 35mpg (with presumably similar co2
emissions). So it's not necessarily related to engine size.
--
Roland Perry