As a regular aspirin user and hence more likely to suffer stomach and intestinal bleeding than the general population, I was recently asked to participate in a long-term medical study aimed at determining whether or not the elimination of a particular gut bacterium known to cause stomach ulcers by the administration of a specific antibiotic would eliminate the risk of ulcer formation in aspirin users.

When the initial tests results are returned and if I am carrying the bacterium in question, I will randomly be given either a course of antibiotics or a course of placebos following which I and all the other participants will be monitored for signs of ulcer formation over a five-year period. At the end of the study, a decision will be made whether or not to offer the antibiotic to all people who are prescribed aspirin by their GPs.

The important point here is that during the trial I will not be told whether I am receiving the antibiotic or the placebo. This is the way scientific trials are carried out to eliminate subjective bias in the sample population.

My point was that one's opinion as to whether or not one fuel is somehow 'better' than another is subject to personal perception and subjective bias and lacks academic rigour. As we know, consumers are influenced in their purchasing decisions by skilful and manipulative advertising and therefore have enhanced expectations about their choices. Obviously, if we are paying 10p a litre more for 'Shell Super Whizzo' than for 'Tesco Boggo Standard' we will expect it to be better and may unconsciously modify our driving to ensure that we do get more mpg or produce less exhaust smoke.

A current television series is evaluating selected households' food buying decisions and replacing their regular purchases with cheaper alternatives in order to save them money. Interestingly, when given some of their usual purchases in plain packaging, many of the people involved cannot identify the bread, breakfast cereal, baked beans and beverages that they consume every day. A common theme is that the subjects of the study look down on 'own brands' as being inferior to the well-known brands because they cost less and are not as attractively packaged. Could the same be true of the fuels that we put in our cars? It's a legitimate question to ask.

It would be very interesting if some of the drivers who are so adamant that they can tell the difference between supermarket and premium fuels were subjected to a similar blind study over an extended period. Their vehicles could be filled by a third party either with cheap or top price fuel but they would not be told which was currently in their tank. Throughout the survey the drivers would be required to record their impressions of each fuelling of their vehicles and then say which fuel they thought that they were using.

I obviously have no way of knowing what the outcome would be but I would be very surprised if the drivers' predictive accuracy was statistically valid, my bet being that it would be as near as damnit to 50:50 and no better than tossing a coin.

Ah, the cut and thrust of intelligent debate.

1. There is no such thing as 'supermarket fuel.' No supermarket chain in the UK owns a refinery. The fuels sold by supermarkets and the big petrol brands come out of the same tanks at the distribution depots, the only differences being the additives put into each compartment of the tanker via a precision metering system at the time of loading. Needless to say, the formulations of these additive packs are closely guarded trade secrets.

2. All fuels sold throughout the Europe Union have to meet strict European quality standards. There are therefore no 'rubbish' fuels on sale.

3. Some people say that they can detect a change in their car's performance depending on which fuel is being used while others say they cannot. However in every case of which I am aware, the driver knew what was in the tank at the time of the 'test'. To the best of my knowledge, there has never been any blind testing to remove subjective bias.

4. In the recent television food programme to which I referred, a woman who was a self-declared baked bean fanatic with a prodigious monthly consumption and who would only ever eat Heinz brand, when presented with samples in unmarked packaging said she didn't like the Heinz ones and much preferred far cheaper beans sourced from Asda. Whether we like it or not, we are all influenced by marketing psychology.

5. All I was suggesting was that if subjected to a blind testing of cheap and premium fuels, drivers similarly may not be able to tell the difference and if anyone was offended by this suggestion or decided to perceive it as a personal criticism then that is their problem for which I make no apology.

6. On the question of tyres (not one that I raised), well, if independent tests of a variety of makes showed that Hong Phooeys were every bit as good as Michelins but at half the price then yes, I wouldn't hesitate to have them fitted to my car. To do otherwise would be ridiculous.

7. When faced with any sweeping generalisation such as "all supermarket fuels are crap," I will continue to ask to be shown the evidence supporting the assertion and not base my convictions on subjective perception and anecdote.