John Hemming's supermarket bill is a little higher these days, after he decided to have his Land Rover Discovery converted to run on vegetable oil.
Mr Hemming, the Liberal Democrat MP for Birmingham Yardley, now visits his local Somerfield store rather than a garage when he needs to top up his tank.
At £8 for a large can, or 53p a litre, he is making a saving on the cost of a tank of diesel.
He is also promoting his party's green credentials by cutting down on fossil fuel emissions.
Mr Hemming paid £1,000 to have twin fuel tanks installed on his Discovery. He still has to start the engine using diesel, but switches once the vegetable oil has reached 80 degrees Celsius.
The practice is perfectly legal, although the MP does have to pay 27p a litre in tax to the Government for the privilege of using oil rather than diesel.
He said: "I will eventually be able to convert to 100 per cent vegetable oil. It's a far more sustainable and environmentally friendly way of running a car because you are not burning fossil fuels in huge quantities.
"The engine performance is spot on and I still get about 35 miles to the gallon. You would never know the difference."
Mr Hemming said he was experiencing problems in organising duty payments.
"I have been chasing the Inland Revenue to pay them, but without success.
"So far I have bought about 60 litres of oil, so I owe them about £16. There may also be a VAT issue. It comes to something when you have to keep asking the taxman to take your money," he said.
Mr Hemming insisted he was not involved in a public-ity stunt but was doing something practical to help defeat global warming.
"Unlike David Cameron, I don't cycle to work while a large car follows behind carrying all my papers and a change of clothes. This is a very effective way to demonstrate that you care about the environment.
"Of course, I do get a few funny looks at the supermarket when they see me buying so much oil. It could be worse, if I lived in Surrey I suppose I would have to buy olive oil."
Several thousand people throughout the country are thought to have converted their vehicles to run on a mixture of diesel and vegetable oil. The practice has been commonplace in parts of Europe, particularly Germany, for years.
Environmental campaigners are urging the British Government to introduce tax breaks in an effort to encourage more conversions.
The Buckinghamshire-based Low-Impact Living Initiative, which helps people to lead more sustainable lives, runs courses on converting diesel engines to run on oil.
The organisation's website explains the benefits: "Vegetable oil is a biofuel and as such is virtually carbon-neutral. This means that the carbon dioxide released when the fuel is burnt is absorbed by the plants that will provide the next crop of oil.
"As CO2 is the main greenhouse gas, switching to biofuels can help slow down global warming."
Switching to Liquid Petroleum Gas would have saved Mr Hemming even more money. LPG costs about 34p a litre compared with 53p a litre for vegetable oil. ..SUPL: