Motoring: Burning the daylight oil

Thinking of buying a diesel? Use your calculator first, says Roger Bell, after a year with an oil burner

Are you doing your bit for the ozone layer? Join the club. I took the eco-friendly plunge a year ago. Goodbye Alfa Romeo, hello Peugeot 306D Turbo. I even persuaded myself I'd be better off. I already admired the 306, with its suspension that absorbs bumps as well as it handles corners. Gripes? Mine leaks (an inch of water in the footwell isn't funny). Otherwise, no serious complaints, so let's talk diesel.

The bad news is that most diesels cost more than comparable petrols, and many still need frequent oil/ filter changes - every 6,000 miles for the Peugeot - which adds to the costs. The good news is that diesels like the 306 should hold their value better than petrol siblings.

You're on safer ground comparing fuel consumption. My 306 averages 47 mpg, which squares with the official Euromix of 46.3. The comparable petrol 1.6 gives only 36.5 mpg, but as derv is no cheaper than unleaded, the savings are modest.

What may trigger another diesel boom is the introduction of advanced direct-injection diesels. The latest car to get DI is Renault's Megane. Parsimony gives diesels a range advantage: some will exceed 800 miles on a tankful.

All the majors are working on DI to improve economy, emissions and performance. But the usual, horsepower-based performance yardsticks - 0-60mph and top speed - are poor guides to a diesel's verve. Seat's 58mpg Ibiza TDi will accelerate from 50 to 70 mph in top faster than an Aston Martin Vantage in the same gear. So will Rover's cracking 220 SDi. My 306 also impresses with mid-range zap. What offends me is the filthy black smoke it belches when I put my foot down.

Diesels are good on some emissions, eg CO2, because they burn less fuel. But dated or badly tuned ones can be very dirty. On all-round merit, VW sets the standard for Peugeot's coming direct-injection motors. The new Golf's catalyst-cleaned 110 bhp 1.9 Turbo gives out no smoke, no diesel smell and virtually no CO or HC emissions. Yet it does 120 mph and 57.6 mpg. Beat that.

Although most diesels suffer from cold-start clatter, the good ones sound no worse than discreetly gruff when under way, and mine is happiest on a motorway.

I do occasionally rue the switch to diesel. Then I'll do a long run, and I'm at peace with the decision. Tempted? First do your sums. If low all-in costs are a priority, consider this: Parker's price guide says the eight cheapest cars to run over a three-year/ 30,000-mile period all run on petrol. Suzuki's Alto heads the list.

Diesel pros ...

More economical than petrol. Extends range between fill-ups. No ignition system to go wrong. Good ones hold value well. Good acceleration from turbos. Unfussy motorway cruising. CO2 reduced.

More expensive to buy. Need servicing more often. Non-turbos sluggish. Fuel messy and smelly. Rough, clattery idling. Less refinement than petrol. Sooty exhaust. Downmarket image.