That's never enough for politicians though is it.
My last car was a big V6. People occasionally commented about paying higher taxes, and looked baffled when you point out that if it really does consume more fuel then the massive tax on petrol means I am already paying higher taxes. Increasing the VED was punishing me for no good reason. I now have a diesel. The difference? I don't like driving as much, fuel bill is slightly lower (25mpg vs 35mpg town driving), I save about £100 on VED per year. Given that after a year I still don't like diesels I would like to trade the car in for another V6, but that would be a post 2006 car now and the VED on those is crippling thanks to it being based on something other than my normal commute.
For those wondering, my commute is only a couple of miles of stop-start across town. If the weather is good, I cycle it. I fill the tank of the car every month or so, the diesel saves me about £15 on that refill. So would I pay £150 per year extra in fuel to enjoy driving and not be sputtering diesel fumes everywhere? Yep. Do I want to pay about another £300 in VED by switching back to petrol? No, so I guess I will stick to stinking the place up and being angry.
I think enough people are struggling to pay their electricity and gas bills as it is, I don't see that they can raise it. They could put a "green tax" on purchasing what they think are inefficient items, but then the really inefficient stuff isn't exactly mass market. I knew someone who had a Musical Fidelity class-A amplifier. The thing cost thousands, chucked out a few hundred watts all day every day as it wasn't supposed to be turned off to keep the amplifier in perfect setup. How much tax would it take to stop someone buying something like that?