You're standing in the showroom admiring two identical cars, and just as the salesman spots you doing a few mental sums, he plays his trump card.
Take the diesel car on the right and you'll get at least 55mpg. Sure, it costs a little more to buy, but just think about how much you'll save at the pumps.
This little drama will be played out in new and used showrooms across the country this weekend, and without doubt lots of peo ple will end up driving the diesel car home.
And for many of them, it will make a lot of sense - particularly if they drive more than 20,000 miles a year. However, for the average buyer who simply wants the best possible car for the smallest outlay, it may well end up costing them more to run.
Sales
It should be pointed out that modern diesels are very good, bearing no relation to the models that were produced as recently as five years ago. The old image of being slow, smelly and noisy has all but gone, and today's latest - common rail - engines offer refinement and performance that in many cases now exceeds that offered by petrol. But the key selling point is their economy - smaller diesel-powered cars now get well over 60mpg and many family saloons average 50.
Compare that to the older petrol car you are trading in, and it can look very attractive. Also, if you are a company car buyer, in most cases you will pay significantly less tax by opting for a diesel-engined model.
And they've certainly been selling. Buoyed, particularly by those last two points, sales of diesel cars have more than doubled since 2000. According to the Society of Motor Manufacturers and Traders, 27% of cars sold in 2003 were diesels, and it is already predicting that the figure will rise to 35% by the end of next year. But despite all this, they're not right for everyone.
Buying new
The principle thing against the purchase of diesel is that they gener ally cost more to buy. Although it varies greatly according to which model you are looking at, diesels cost on average 10% more than their petrol equivalents. In most cases this equates to at least £1,000 - much more at the top end of the spectrum. For example, the Mondeo, Britain's best-selling diesel car, costs roughly £1,500 more than its petrol equivalent (before discounts). It probably offers an no more than an extra 10mpg, which means you've got to be doing close to 20,000 miles a year to recoup the extra outlay - assuming you keep it for three years. And remember, petrol engines are more fuel-efficient than they were a decade ago.
Things are clouded further when you consider other costs. Depreciation, the number one cost of car ownership, is now almost identical in percentage terms for most diesel and petrol cars (it used to be much better for diesels) so in real terms you are losing a little more. The other factor is servicing. Again, it really varies according to the model, but some diesel cars cost an extra £100 in annual servicing. Add in the cost of the extra finance and the fact that you're sliding around diesel-covered forecourts at refill time, and it's starting to take the shine off the deal.
"People really need to look at the real-life costs over the period they intend to keep the car, rather than being blinded by good mpg figures," says John Maslen, editor of Fleet News, the bible for those who run large company car fleets.
"Unless they drive in excess of 20,000 a year, private buyers really need to do their homework before opting for a diesel - in fact any car. It will all depend on what deals are being offered at the time. Ironically, so many people are now buying diesels that some manufacturers are having to further discount the petrol models to boost their sales."
Buying used While the cost savings are marginal when buying new, the difference is dramatic when purchasing second-hand. Used diesel cars command an often ridiculous price premium, which should put off all but the most serious of mileage accumulators. Those canny enough to buy two- to three-year-old cars will probably find that their money will get them into a better or newer model, with a lower mileage, if they avoid the diesel option.
A look at any car supermarket website will show the dramatic difference in prices for petrol and diesel cars. At one of the UK's biggest, Car Giant, diesel models currently cost between 10 and 20% more than their petrol equivalents.
By the time you have factored in any extra costs of ownership, you probably have to be doing upwards of 25,000 miles a year to make a diesel purchase cost-effective.
Plus points
It should be noted that there are some exceptions to the above. Diesel cars are much better if your vehicle spends all day in congested cities, or lives entirely on a diet of short runs. Petrol engine fuel consumption suffers dramatically when sitting in stop/start traffic, and if your driving fits that pattern, a diesel's best. Insurance costs can also be lower.
Lastly, it should be said that lots of people now prefer driving diesel-engined cars and say they would never go back to petrol. If that's the case, fine - just don't expect to be saving any money.
Crunch the numbers
For average mileage drivers (12,000 a year) try not to pay a premium for the diesel model when buying new.
Buying nearly new, you will almost certainly be asked to pay a premium, does your annual mileage really justify it?
Establish what the servicing costs will be and work out how much rival models will cost to maintain during your ownership.
Fleet News' website, www.fleetnewsnet.co.uk, shows running costs for every car, both petrol and diesel, in pence per mile - you might be surprised at the figures.
