Diesel sales at their highest point

• Fuel economy concerns drive diesel sales says SMMT
• For many, diesel doesn't save cash, however
• Break-even mileage on a Focus is 40k+ miles

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More motorists than ever bought diesel cars last month in the search for greater fuel efficiency, but they may not be saving money as a result.

The Society of Motor Manufacturers and Traders reckons fuel economy and environmental concerns pushed diesel sales to their highest point so far this year, with four out of every 10 cars hitting the road running on the fuel.

If motorists are hoping to save money at the pump by buying diesels they need to do their sums very carefully, however.

Many motorists may not cover enough miles within a typical three-year ownership period to ever pay off the price premium diesels hold over their petrol counterparts.

Motorists would have to cover around 40,000 miles before they even started saving money in an entry-level diesel Ford Focus, for instance.

Compare our favourite five-door LX-trimmed Focus, a 1.6 16v petrol, with its TDCi diesel counterpart and the break-even mileage shoots up to more than 65,000 miles.

Our choice in the Vauxhall Astra range, second to the Focus in the sale league, is the 1.6 petrol in Club trim. Buy a slower 1.4 diesel and you'll still have to travel more than 50,000 miles to start saving.

If you want similar performance to the petrol, however, you'll have to spend even more on a 1.7 diesel and see 80,000 miles pass by before you put a smile on you bank manager's face.

How to work out the break-even mileage point
• Work out the diesel price premium - deduct the price of the petrol alternative from the diesel option

• Calculate the cost per mile for each - divide the cost of a gallon of the appropriate fuel by the combined fuel consumption of each car you're considering (you can find the AA's average fuel prices for this calculation here)
• Find out the cost per mile difference - subtract the diesel cost per mile from the petrol figure to see how much cheaper it is
• And hey presto - divide the price premium by the cost per mile difference for the result