SUVs represents roughly half of all light vehicles sold by domestic US automakers, yet they achieve just 20.7 miles to the gallon on average. Carmakers will now be forced to tackle this issue, but the required 1.5 mile per gallon increase may not be ambitious enough.

The National Highway Traffic Safety Administration in the US has imposed new rules requiring automakers to raise the corporate average fuel economy (the average fuel consumption across a vehicle manufacturer's entire range) for light trucks by 1.5 miles per gallon (mpg) between 2005 and 2007. This represents the largest increase to sport-utility vehicles (SUVs), pickup trucks and minivans in 20 years.

The carmakers feel meeting the increase will be challenging, and that American buying habits will need to change. Some also fear that new regulations will benefit foreign competitors, as they sell more passenger cars than their American counterparts.

However, environmentalists argue that compliance should be easy given the technologies at the manufacturers' disposal. It is estimated that the total cost of the improvements to the industry could exceed $US1.5 billion.

A comparison with the fuel economy of vehicles sold in Europe gives a clear indication that the current levels in the US are unacceptable. Sales of diesel cars in Europe have been increasing rapidly for some time and an increasing number of cars are able to average more than 50 mpg.

Environmentalists and citizen groups are pressing for even more stringent fuel-economy standards than the 1.5 mpg set by federal regulators. Indeed, tougher regulations would reduce US dependence on foreign imports and some automakers are already offering more ambitious targets. Jason Mark, head of the clean vehicles programme for the Union of Concerned Scientists, estimates that Ford, for example, will be able to improve by 1.8 mpg in the target period.

As well as diesel technologies, carmakers can also make use of hybrid vehicles. These combine electric motors with a petrol engine, and "displacement-on-demand" technology, which shuts down half of the engine's cylinders at motorway speeds.