What Car? Q&A - When is it worth buying a diesel

Q: Are there any base calculations as to when petrol cars become financially more viable than diesels?
Ian Seymour

A: The easiest way to compare the cost of ownership is to look at the pence-per-mile figures listed in the back of What Car? each month or in the specifications of road tests online at Whatcar.com.

These figures include depreciation, which is likely to be a little slower on a diesel, servicing, which is likely to be a little more expensive, fuel and road tax.

If you want to work out a rough break-even point for a diesel against a petrol – that is, when you pay off its price premium and start to save money – you’ll need to break out your calculator.

First calculate the simple fuel-cost-per-mile figure by dividing the combined fuel consumption figure of each car into the price for a gallon of fuel (currently £4.00 for petrol and £4.19 for diesel).

Subtract the resultant diesel figure from the petrol one, which should be larger (unless you’re looking at very different options), and divide this into the price premium to obtain an approximate break-even point.