Tax cuts introduced by the government for environmentally friendly vehicles were meant to spur sales of fuel-efficient models and help fight global warming.

Since the system got under way in April, however, carmakers have found a loophole that allows them to take advantage of the eco-car tax cuts for models with lower fuel efficiency, even though more economical configurations of the same cars are missing out.

In some cases, fuel-hungry cars are selling better than more efficient varieties.

The reason for the anomaly can be found in the nine weight categories used for the tax cut system. The higher the weight category, the less stringent the fuel efficiency standards a car must meet to receive tax cuts. If a vehicle is fitted with extra parts, it may be classified into a higher weight category and, as a result, be subject to tax cuts.

The discrepancy can be seen in sales of the new model of Toyota Motor Corp.'s mainstay sedan Mark X, which went on sale in October. The car comes in 10 versions, which differ by the size of engine displacement, fittings installed and other factors.

Toyota received orders for about 14,000 units in the first month of sale. Of those, about 80 percent were for two of the 10 configurations--both less fuel efficient, but ultimately cheaper due to the fact they are the only two eligible for tax cuts.

The 2.5-liter cars are equipped with special electromotive devices that can adjust the position of the front passenger seat. This addition also adds 10 kilograms to the weight of the vehicles, elevating them to the next weight category.

The two cars run 12.4 km per liter of gasoline--0.6 km less than comparable varieties not installed with the electromotive devices.

The other models weigh 1,510 kilograms. In the category to which they belong, they must run at least 15 km per liter to be eligible for the tax cuts. In the next category up, however, cars only have to run 12.1 km to the liter to be entitled to tax cuts of about 100,000 yen.

Though one of the two most popular cars costs 52,500 yen more than those without the devices, it is ultimately about 50,000 yen cheaper owing to the tax reductions.

A Toyota-affiliated dealer in the Kanto region admitted: "We're promoting (the two types) by saying that, thanks to the tax cuts, customers can get the special devices at bargain prices."

Other carmakers are advertising their vehicles in the same way.

In an explanation of the eco-car tax cut system provided on its website, Nissan Motor Co. shows how customers can receive 75-percent tax cuts by installing optional parts on some types of the minivan model Lafesta that are not ordinarily subject to tax reductions.

The parts include curtain air bags, which are installed on both sides of the vehicle to protect both the driver and passengers from side-on impact.

In September, Fuji Heavy Industries Ltd. began to sell special variations of its midsized sport utility vehicle Forester 2.0X, equipped with sunroofs and other parts to boost their weight. Though the additions reduced their fuel efficiency by 1.2 km per liter, the extra heft made them eligible for tax cuts and, as a result, they accounted for 20 percent of all Forester vehicles sold in October.

Carmakers are up front about the situation.

"Some customers bought new vehicles to get the tax cuts. (Even though the cars are less fuel efficient than other variations), they are still better than their previous vehicles," an official of Toyota's public relations office explained.

"It's a matter of course for salespeople to recommend cars that offer advantages to customers," an official of Nissan's public relations office said.

However, a senior official of a major carmaker said, "It's a flaw in the (eco-car tax cut) system, and it has caused a deviation from the original purpose of the tax cuts."

The government introduced the tax cut program not only to promote sales of environmentally friendly cars but also to encourage spending by consumers who have been spooked by the global financial crisis.

"As the two factors are mixed, the purpose of the tax cuts has become unclear," said Kei Horai, a researcher of SC-ABeam Automotive Consulting.

To promote eco-cars, he said, the government should stop classifying vehicles by weight, and introduce a system like the one in France that fines vehicles with poor fuel efficiency and rewards those that are more economical.

An official at the Ministry of the Land, Infrastructure, Transport and Tourism said it introduced the weight-based system to address a variety of needs among users (including those who favor large vehicles or minivans).

"If there are trends in buying that are undermining the purpose of the tax cut system, then we will investigate them," said the ministry official, who belongs to the Environment Division of the Road Transport Bureau.