It is now more important than ever to keep an eye on the potential used value of a car when buying a new one.

Future resale value is a pertinent subject when the country is suffering an economic downturn and drivers are faced with increasing motoring costs.

There’s now the very real fear in the UK car market that not-so-eco-friendly models could face being shunned by buyers just three years after being driven, brand new, out of the showroom.

September has been the key month for the last seven years for new car registrations, following the introduction of the new style number plates, but here in 2008 the credit crunch looks set to take its toll on the number of new 58-plate registrations.

Many motorists are putting the decision to buy a new car on the back burner, so what do the experts think are the current best buys when it comes to resale potential?

Leading car data company HPI has put its neck on the line by naming cars that will be top used buys, ones that will not only be more cost-effective to run and environmentally friendly, but are most likely to hold their value.

“Those keen to trade in their current vehicle for a new, or newer, one needs to think carefully about their next purchase,” says Martin Keighley, HPI’s used car valuations expert.

“Winners will be anything small and economical, with low CO2 ratings. My top picks include the Toyota Aygo and its sister cars, Citroen C1, Peugeot 107, Fiat 500, and Hyundai i10,” he says.

“Image-conscious drivers will plump for the Mini, which is a bullet-proof buy as long as they stay away from the high-end models.”

Keighley says that when it comes to medium-sized cars, eco-friendly models such as the VW Golf BlueMotion, Audi A3 1.9 economy models, and Seat Leon 1.9 Ecomotive are all sound investments.

“With money tight, many drivers have been panicked into downsizing to cut fuel and Vehicle Excise Duty bills. As a result, the list of undesirables is growing, with large petrol engines top of the no-no pile,” he says.

“I would even consider that, these days, the 2.0 is too large in 4x4 people carriers or Vauxhall Vectra-sized models. As times get tougher we need to be aware of all the factors that impact on vehicle values.

“Although common sense has a lot to do with buying decisions, don’t forget desirability can also rule the day.”

Meanwhile, Fiat is currently performing better than a host of other car makers in the used car market, helped greatly by the current trend to downsize, according to the experts behind the famous Glass’s Guide to Used Car Values.

Publisher EurotaxGlass’s says Fiat has benefited more than many rivals, simply because a significant proportion of its current model range is comprised of small, fuel-efficient vehicles.

“Fiat is well-placed within the current market, because the majority of its more recent offerings are in the small car sector,” says Jeff Paterson, chief car editor at EurotaxGlass’s.

“The Punto and Panda have clearly been on used car buyers’ shopping lists, especially now perceived value-for-money is having a greater influence over purchasing decisions.

“The Fiat 500 also enjoys continued success, and those few examples which have found their way onto the used car market are in high demand, with prices not falling far behind list.”

That said, sales of Fiat’s larger models are also following the wider market trend, says Paterson. “With consumers preferring smaller vehicles, interest in Bravo has been disappointing, and those looking to sell or part-exchange a Ulysse MPV will find its value has fallen sharply over recent months.”

Finally, the price of fuel, road tax changes, and credit crunch worries have all combined to prompt serious concern among the 4x4-owning public about whether their 4x4s are going to depreciate heavily.

Others, fuelled by industry rumours of massive drops, are hoping for the opportunity to pick up a bargain 4x4 whose running costs can be offset by the huge saving on the purchase price.

According to the latest Used Car Expert magazine, the market currently shows a mixed picture so owners are advised to proceed with caution before trading their 4x4.

Used Car Expert magazine editor, Matthew Tumbridge, says when looking at two-year-old cars, it becomes clear some 4x4s are holding up better than others.

“But, with the uncertain market, there are no rules of thumb for avoiding heavy losses. Consumers really need to do their homework at the moment if they want to play depreciation to their advantage,” he says.

“If selling an Audi Q7, for example, the petrol version is going to lose you a lot of money. But, if selling a Mercedes M-Class, the losses on the petrol are almost painless.”

Tumbridge points out that buyers can find almost giveaway prices on petrol engine Mitsubishi Outlanders, but because Jeep Grand Cherokees don’t come with a relatively small petrol engine any more, the older cars have become more popular.

“This has firmed up the prices on 2003 and 2004-year cars, and 2006-2007 diesels are losing value,” Tumbridge says.