The emission-cheating scandal that is engulfing Volkswagen is disturbing on so many levels.

But the motoring fraternity should not be too surprised really, given the incredible efficiency figures touted by car manufacturers in recent years.

Today, a European compact will supposedly cover 100km on about five litres of petrol - a figure that was achievable a decade ago only by hybrids.

Even a sports car with more than 500 horses and four seats is said to be capable of running for 100km on less than 10 litres of fuel.

Most of today's declared figures are unachievable in the real world, of course. According to European lobby group Transport & Environment, the average gap between factory-stated consumption and actual consumption was 40 per cent last year - up sharply from 8 per cent in 2001.

A number of countries have fined carmakers for over-stating their fuel economy figures. Last year, Seoul fined home-grown Hyundai Motor one billion won (S$1.2 million) for over-stating the economy of its Santa Fe SUV by 8 per cent.

Audi, Volkswagen, Chrysler and BMW Group were likewise fined about one million won each.

Hyundai, Kia and Audi have also been fined in the United States for inflating fuel economy claims.

Early this year, Italian consumer group Altroconsumo filed a writ of summons against Fiat-Chrysler and Volkswagen for the same claims.

In my experience, fuel consumption figures of European brands are the most exaggerated. Those from Japanese makes are the least inflated. Perhaps the Japanese testing mode is more reflective of driving conditions in Singapore.

If the Singapore authorities were as stringent as their counterparts in South Korea or the US, many manufacturers would have been fined here too.

And Singapore should take a harder stance, since fuel economy has a direct bearing on carbon dioxide emission, not to mention toxic pollutants such as particulates and nitrogen oxides (NOx).

Our Carbon Emissions-based Vehicle Scheme (CEVS), introduced on Jan 1, 2013, grants tax rebates and imposes tax surcharges on cars based on the amount of CO2 they produce. Within the first two years of implementation, the scheme cost the Government $62 million.

A company that over-states its fuel economy is in effect understating its carbon emission. Since this concerns tax revenue, shouldn't the authorities take a more serious view?

Singapore Customs conducts audits on car importers to make sure they are not under-declaring open-market values (OMV). Under-declaring OMV is a form of tax evasion because tariffs such as GST and Additional Registration Fee are based on the OMV.

The Volkswagen saga revolves around significantly higher amounts of NOx emitted by about 11 million cars across the group's brands.

While the CEVS does not take NOx into consideration, the fact that the affected VW cars produced as much as 35 times the declared amount of the noxious pollutant indicates that their CO2 levels are likely to be higher too (although the link between CO2 and NOx is not as direct as the one between CO2 and fuel economy).

Also, seeing how Japanese economy figures are more realistic in the Singapore context, perhaps we should have adopted Japan's emission standards instead of Europe's.

Japan's emission regulations have separate offshoots that focus on nitrogen oxides and particulate matter as well. In a dense urban environment, these two pollutants are arguably more relevant than CO2 because they impact health directly.

This is why diesel cars - which produce less carbon but more NOx and particulates - are not popular in Japan. Instead, the country focuses on hybrid and electric models to reduce its carbon footprint.

The European standards specify NOx and particulates too. But going by how their fuel economy figures tend to be off the mark in the kind of driving we do here (largely city driving), questions arise.

Also, the exhaust treatments used in newer diesel cars to reduce the amount of NOx released into the atmosphere work at fairly high temperatures. These temperatures are sometimes not attainable in city driving. Journeys are over before the system can warm up.