Cars that use regular fuel but are filled with so-called cleaner premium gas could be harming the environment, says at least one Canadian auto expert.

A CBC Marketplace investigation found many Canadian drivers are paying more for premium gas, believing it's better for their cars and the environment. But Marty Stanfel says unless you own a high-performance car, premium gas could be hard on the wallet as well as the Earth.

Auto expert Marty Stanfel says most drivers can stick to regular gas for their cars.

"It’s actually hurting the environment," said Stanfel, an instructor at the Canadian Automotive and Trucking Institute. "Whether or not you’re cleaning your valves and cleaning the inside of the engine, I couldn’t say that’s true. But I do think you’re hurting the environment now by running the premium fuel in a regular-fuelled car."

Many of Canada's biggest fuel retailers, including Petro-Canada, Shell and Esso, all sell a premium fuel that can improve engine performance and lower emissions. According to Petro-Canada's website, its premium brand SuperClean is "an environmentally responsible choice" that can lead to a 15 per cent emission reduction.

But the high-performance, environmentally friendly marketing message can overshadow the fact that premium fuel is only meant for certain cars, and using the wrong fuel can have negative consequences.

Putting premium to the test

In a test conducted for Marketplace, Stanfel fuelled up a 2013 Chevrolet Cruze — which is designed for regular fuel use —with Shell's regular, then premium V-power gas, and simulated identical drives with each. He found the high octane fuel didn’t burn efficiently and produced extra hydrocarbons, compounds that contribute to the formation of greenhouse gases.

'If they use the regularly priced gas … they’d be saving money as well as doing a favour to the environment.'—Marty Stanfel, Canadian Automotive and Trucking Institute

Stanfel says most drivers can ignore the marketing, because the cheapest gas is also the best one.

“If they use the regularly priced gas… they’d be saving money as well as doing a favour to the environment,” he said.

Toronto resident Karuna Brandy, who owns a Honda Civic, used to buy premium fuel, believing it's better for her car and the environment.

"I’m a yogi eco person," she told Marketplace. "I don’t know if it’s true or not, but in my mind, I believe it’s true that if I buy the higher-grade gas, it burns cleaner."

She was shocked when she learned the results of the testing.

“I'm speechless," she said. “Now I’m driving my car knowing it has premium gas in it, wondering if my emissions are higher than if I was driving my car with regular gas.”

Karuna Brandy was "speechless" when she learned the premium gas she buys can be dirtier than regular. (CBC)

Brandy simply changed her buying habits, but some disgruntled drivers have gone further.

In 2001, Illinois resident Mark Oliveira went all the way to the state Supreme Court with a case against Amoco Oil Company, arguing Amoco violated the state’s Consumer Fraud and Deceptive Business Practices Act. He charged that Amoco used misleading ads claiming its premium gasolines would improve engine performance and benefit the environment. Oliveira was seeking to open the door for a nationwide class action against Amoco, but the court ultimately disagreed.

Do your homework

Gas companies aren’t trying to mislead anyone, insists Carol Montreuil, vice-president of the Toronto-based Canadian Petroleum Products Institute.

He says drivers just need to do their homework.

“One has to look at the driver’s manual,” he said. “It’s very clearly spelled out if you need [premium gas] or not.”

Shell, Husky, Esso and Petro-Canada all declined interview requests for this story, but included similar advice in their responses.

"As each engine is different, we recommend that car owners refer to the manufacturers' recommendations in their manual when determining whether to purchase premium fuel," read an email from Petro-Canada.

“I think that at the end of the day, it’s not the research that should decide — it’s the consumer with the performance they’re getting with their car,” Montreuil added. “No one is forcing anyone to buy this premium gasoline.”

Watch Marketplace's episode, Pump Fiction, Friday at 8 p.m. (8:30 p.m. in Newfoundland and Labrador) for a fresh perspective on premium gas.