CNG over there costs about 68 sen (28 cents) a litre, compared to the highest $1.49 per kg here. The kiosks charge different rates in Singapore.
A litre of CNG weighs about a kg.
Mr Heng, 43, reckons he saves about $200 each month by topping up CNG for his Hyundai Sante Fe in Johor Baru twice weekly.
A full tank of CNG costs him about $6 in Malaysia, compared to about $30 here.
Mr Heng, the managing director of parallel import company Cars Motor Group, said his car can travel about 300km on a full tank of CNG.
It would cost him about $60 in petrol to cover the same distance.
He spent about $4,600 to install the CNG kit in his four-year-old car about seven months ago.
Mr Heng headed to JB for CNG because, at the time, there was only one CNG kiosk here at Jurong Island.
Two more kiosks opened at Jalan Buroh and Mandai in February.
'It was just too troublesome because there was only one kiosk then. So, I decided to go to Malaysia to try my luck. I had only a map and it took me close to an hour to find that kiosk (in JB),' he said.
Today, it takes him just 10 minutes to drive 12km from JB to the Petronas CNG kiosk there.
The price difference is big enough for Mr Heng to brave the unpredictable Causeway jams twice a week for his CNG top-ups.
CNG is cheaper in Malaysia because it is produced there and it is also heavily subsidised.
Mr Heng said: 'It's worth my while to top up gas in Malaysia. But it is better now that there are more CNG stations in Singapore. Sometimes when I am pressed for time, I will just top up here.'
The number of CNG cars here is still small compared to petrol cars, but it is growing.
MORE CNG CARS
As of last month, there are 757 CNG private cars, according to latest figures from the Land Transport Authority (LTA).
This is more than double the number in January, with 265 registered CNG cars.
There are more than 528,000 private petrol cars here as of last month.
With the growing popularity of CNG cars, perhaps it is only natural that more motorists here will head north for cheaper gas.
Especially since the three-quarter fuel tank rule imposed on Singapore vehicles crossing into Malaysia does not apply to CNG vehicles at present.
Pump attendants at two separate CNG kiosks in Johor Baru told The New Paper that they had noticed more Singapore cars making weekly trips there to top up CNG.
The attendants said they see about three to five such cars every week, mostly over the weekend.
One attendant, who declined to be named, said in Malay: 'I see one or two Singapore cars every few days. It's not frequent but I do see more of them here on weekends.'
CMelchers, which has fitted over 500 CNG kits here, said about one in 10 of its customers go to Malaysia for cheaper gas.
Business development executive Shannon Sim said the quality of gas in both countries is almost similar, so safety is not an issue.
He said: 'The gas there is very cheap, that's why some motorists here will bear the inconvenience of driving there to top up.
'But few motorists know about the location of these CNG kiosks and they tend to be farther from the Causeway compared to the petrol stations.'
But not everyone is tempted by the cheaper CNG. They feel the cost savings are not worth the effort of going across.
Mr Chew Kok Hua, who drives a Mercedes E200, pumps about $30 worth of CNG at least once a week at Mandai.
After doing his calculations, he concluded it may not be worth his while to pump in JB.
He said in Mandarin: 'I will save at least $20 each time, but it may not make much sense because of the travelling time and traffic jams along the Causeway.'
This story was first published in The New Paper on June 27, 2008.