While confirming that a brand new Tesla Model S would have qualified for a "green" rebate, the Land Transport Authority (LTA) said it was re-examining the case of a used electric car that was issued a $15,000 carbon surcharge.
The car - the first and only Tesla Model S here - was imported by Mr Joe Nguyen, a 44-year-old senior vice-president with an Internet research firm.
The LTA yesterday said that while all imported used cars must be tested individually, it is working with Tesla engineers to see if the Model S - which qualifies for tax breaks in most countries - was tested correctly in the first place.
The latest turn of events came just days after Tesla boss Elon Musk called Prime Minister Lee Hsien Loong after news of the Model S' seemingly anomalous surcharge reached him.
The Prime Minister's Office told The Straits Times that various agencies are looking into the case.
The car was first registered in Hong Kong in 2014, and had clocked about 1,000km before it was shipped here.
The LTA said tests conducted by the Vicom Emission Test Laboratory showed the car had a power consumption of 444 watt-hour/km.
After factoring in carbon dioxide emissions of 0.5g/watt-hour at the power plants, the car's emission was deemed to be 222g/km. This puts it in the $15,000 surcharge band under the Carbon Emissions-based Vehicle Scheme (CEVS).
Tesla Motors has informed the LTA that the Model S' energy consumption was rated at 181 watt-hour/km when it left the Tesla factory on June 28, 2014.
A brand-new Model S would have qualified for a $30,000 CEVS rebate, the LTA noted.
But Mr Nguyen's car was not new. All imported used cars have to be tested individually since their efficiency can vary, depending on "how they were maintained".
The LTA said it "would not know how much the car's condition might have deteriorated" otherwise.
"We cannot make exceptions as it would not be fair to other car owners," it added.
Tesla spokesman Atsuko Doi said the emission of a new Model S is less than half that of a Mercedes-Benz S500L, which produces around 200g/km of carbon dioxide.
If oil extraction, distribution and refining were included, the S500L's carbon emission would be almost three times the Tesla's.
Ms Doi said the company is in discussions with the LTA "to ensure a proper understanding of these issues, and to make sure that they are correctly testing our customer's Model S".
She added: "We are confident that this situation will be resolved soon."