Private-hire cars clock significantly higher mileage than normal passenger cars and may be contributing to congestion here.

According to a list of cars put up for sale by Uber-owned Lion City Rentals, they average 2,145km per month - 54 per cent more than the national average here for passenger cars at 1,392km.

The data is culled from 32 cars that Lion City Rentals is auctioning off. They range from six to eight years old, with the majority having clocked some 2,000km per month.

The lowest mileage was that of a Honda Airwave, which clocked around 1,312km per month. The highest was by a Honda Civic which covered 3,074km per month.

In comparison, taxis clock mileage between three and seven times that of a regular car, or about 4,000km to 9,000km per month.

From a traffic standpoint, the data seems to support suspicion that the private-hire industry's explosive growth is contributing to congestion. Since Uber and Grab landed here in 2013, the number of cars offering taxi-like services has grown from zero to more than 40,000 - eclipsing a taxi population of around 25,700.

Indeed, studies in American cities such as New York, San Francisco and Seattle suggest that Uber and rival Lyft have contributed to worsening congestion. Both firms have refuted these observations.

Transport experts say it is hard to pinpoint the causes of congestion, as there are many - from roadworks to weather to rail breakdowns. Yet, they are not surprised private-hire cars are clocking such high mileage.

Singapore University of Social Sciences transport researcher Park Byung Joon said: "You don't have to look far. Each taxi easily clocks more than 100,000km per year. That is a lot of traffic volume created by one car."

He added: "I am a sceptic of the notion that private-hire vehicles are part of the sharing economy."

There is no sharing as it is a paid service.

National University of Singapore transport researcher Lee Der-Horng said: "Based on my understanding of Beijing, its overall traffic came down after private-hire vehicles became a widely subscribed way of travel.

"In Singapore, I do notice the surge of private-hire vehicles has caused some localised congestion at places like shopping malls and office buildings."

He said it may be necessary for building owners and the regulator to come up with pick-up and drop-off points for such vehicles, "just like taxi stands".

Some motorists are convinced private-hire vehicles are contributing to jams. "Since the car population has fallen, I would expect traffic to improve. But I find congestion has worsened in recent years," said businessman Desmond Koh, 58.

Meanwhile, used car traders say it is difficult for them to buy rental or private-hire vehicles because of their high mileage and a recent influx of such vehicles for sale. "How to resell? Nobody would want because of the high mileage," said one who requested anonymity because his firm supplies cars to Uber.

According to Chinese daily Lianhe Zaobao, Uber's Lion City Rentals sold up to 1,000 cars in the last two months - some as new as a year old.

Asked why it was offloading so many cars, an Uber spokesman said: "As part of its regular operations, Lion City Rentals regularly refreshes its fleet inventory, so that hirers can choose more of the models they prefer. This should not in any way be construed as an indication of the state of Lion City Rentals' business."