There’s no doubt that there are unexpected ambient effects that can suddenly hit your car’s fuel economy on any given day, typically on a familiar journey, and leave you puzzled as to why the mpg figures don’t read any better. When you drive through standing water, at any speed, you normally feel that extra resistance compared with a dry or damp road, because the engine is working harder and using more fuel to help the tyres clear the water away. On seriously wet roads you would probably slow down, which will then be as economical as driving at 10 to 15mph faster on a dry road. But rain is often accompanied by increased winds so, if you are driving into the wind, or buffeted by a side wind, you may use more fuel in such conditions. Steady head winds are not always that obvious, and even a modest 10mph head wind when cruising at 65mph is much like cruising at 75mph, which can make an engine up to 15 to 20 per cent thirstier. If you’re in no great hurry, slow down a bit, and enjoy the return journey when you may have a nice tail wind and get great fuel economy.

But air temperature, pressure, and altitude can also have significant effects on fuel economy. At 25 degrees Celsius, the ambient air is 4 to 5 per cent thinner than at 5 to 10 degrees Celsius, and the aerodynamic drag is proportionately lower. So you could well expect a car to be 3 to 4 per cent more economical on the motorway on a warm summer day than on a cool morning. But what about really hot days, when temperatures can hit 35 degrees Celsius and more, particularly if you’re on holiday in Europe? Research findings suggest that diesels use less fuel when the boost intake air temperature rises in relatively low engine load conditions, with the optimum intake air temperature for a diesel engine around 35 to 40 degrees Celsius. But any intercooler that struggles to cool the boosted air to below 40 degrees Celsius can lead to higher fuel usage and engine stress, particularly if you’re driving hard, maybe towing a caravan or boat. But most engines are designed for far hotter conditions than we experience in the UK and, at normal power output levels, most standard intercoolers are more than adequate. As owners can testify, the original 89bhp 2.0-litre HDi CitroÎn Xsara Picasso, without any intercooler, is quite capable of turning in impressive fuel economy when driven on a light throttle.

Enough of air temperature ñ what about atmospheric air pressure, which frequently varies by as much as 5 per cent, and which also falls rapidly with increasing altitude? Well, the air density doesn’t change too much at UK altitudes, but take your car over a 2,500 metres high pass in the Alps on holiday and the air density typically drops by over 20 per cent. You would notice the lack of oxygen in each lungful of air if you tried to do any physical work but, up to a point, your diesel car’s engine should notice little difference, as a diesel engine’s ECU will instruct the turbocharger to compensate, by blowing that little bit harder. Petrol engines (without turbochargers) can suffer from serious power loss though, at high altitudes.

Normal humidity variations have little effect on diesel engines, but there are possible influences on fuel economy from many other less obvious ambient air conditions. Heavy traffic, particularly large HGVs, creates serious air turbulence that increases the aerodynamic drag, and a poor set of fuel economy figures after a long run on a very busy motorway can be easily explained. It’s a good reason for keeping well clear of such big vehicles whenever you can, particularly ahead of you, but also to either side.

If you’re puzzled why your car seems to be using lots of fuel, although the engine sounds fine, it’s always worth pulling over to check your tyres out, because under-inflated tyres or even badly tracked steering can eat up fuel at higher speeds, and also damage your tyres. Or, if you see the trip or running mpg suddenly falling, it could be that your engine is regenerating its DPF (diesel particulate filter), which can make a big difference to your trip mpg on shorter journeys, although the long-term effect is usually only small, as the ìactive regenerationî is typically performed for only around 5 to 10 minutes in every 10 to 15 hours of engine operation.

Victor Harman