All of the options looked at previously have required you to purchase an new car. So what are the options if you want to keep your existing car?
You could convert your car to run on LPG, this is taxed at a lower rate the petrol and diesel and produces less CO2 but it's not as fuel efficient so your MPG will drop a little and a conversion will cost around £2000. Unless you do a lot of mileage it could take some time for the conversion to pay for itself.
If you have a petrol car you could use an ethenol blend, if you drive a diesel you could use bio-diesel. You will have to find a filling station that has your desired fule which may currently be dificult depening on where you live. The theory with both these fuels is that growing the raw material consumes CO2 so when you burn the fuel you are only releasing what has previously been absorbed. Fine in theory but who much energy and therefore CO2 is produced in the processing of the fuel itself.
