DIESEL-engined off-roaders make a sort of sense: they're more fuel-efficient than their petrol counterparts and, while they do not have the brake horsepower or straight line speed of their stablemates, they tend to have the advantage in lb-ft torque - the low engine speed pulling power that makes all the difference to trailer towing and rut dodging. But since the Government started penalising oil-burners in extra fuel duty on environmental grounds, there's little in it if a diesel is chosen simply to reduce running costs.

Jeep's flagship Grand Cherokee now comes with a 3.1-litre turbodiesel to replace the four-cylinder 2.5-litre unit, developed for the European market, where in most countries other than Britain, diesel fuel is significantly cheaper at the pumps than petrol. Capable of 21.4mpg on the combined cycle, the new Italian-made five-cylinder 3.1 TD is almost 5mpg better off than the ú29,995 Grand 4.0 and 7mpg less thirsty than the range-topping ú34,995 4.7-litre V8. But with the average price of a gallon of diesel at ú3.53 and unleaded petrol at ú3.42, the economy advantage over 60,000 miles comes down to ú790 in favour of the oil-burner. But take away the ú1,000 new price premium and the 4.0 petrol wins by ú210.

In the turbodiesel's favour are fewer carbon dioxide emissions - and therefore vehicle excise duty savings and lower benefit-in-kind charges - and the diesel engine grunt preferred by Europeans who drive off-road for a living or hobby. The 2.5-tonne 3.1 TD produces a relatively puny 138bhp at 3,600rpm, but its torque figure of 283lb-ft at 1,800rpm is not far short of the 220bhp V8.