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I read your post on octanes for high performance engines. I have a Mercedes and it requires 91. I have been mixing 87 and 94 together when my tank gets to half full. Do you think this is okay? – Michael in Toronto

What do you get when you mix regular with super premium? No real help to your car or wallet, says one car expert.

"My advice would be to run one octane or the other," says Stephen Leroux, professor in the school of transportation at Centennial College in Toronto. "I see no benefit in mixing."

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Higher octane gasolines are designed to burn more slowly and produce a uniform flame propagation in your engine's cylinder – they're meant for inside a high compression engine where regular gasoline might start burning before it's supposed to and cause engine knock.

Premium gasoline costs a lot more, but, although it doesn't have ethanol and might have a few added detergents, it's not better.

Mixing the grades to get 91 or so shouldn't hurt, but it probably isn't saving you much money.

Using anything higher than your manual recommends probably won't help your car's performance – but using lower might save you a few bucks, Leroux says. If you notice knocking, you should switch back.

"Try running 87," he says. "If it performs adequately, with no pre-ignition or detonation issues, then there will be some financial savings at the pumps."

Most cars sold in North America are designed to run on 87, even when manuals recommend a higher octane, Leroux says.

If your manual recommends premium (91), it will usually say that you can use a lower grade but should switch back if you get pinging or knocking. Often, it will recommend sticking to higher octane in really hot weather or if at higher altitudes.

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"Remember, this 'premium' recommendation is not coming from your mother, but rather an auto maker looking for marketing-friendly bragging rights," says Consumer Reports deputy online editor Jeff Bartlett in a blog on the magazine's site.

Car companies, some experts and Consumer Reports itself say you should use what the manual recommends – the site notes that Bartlett's view is personal.

If premium is recommended, rather than required, it means that you can use regular – but you'll lose a little horsepower.

How much? Bartlett says the 2013 Ford Escape with the turbocharged 2.0-litre engine produces 240 horsepower at 5,500 rpm on premium fuel and 231 horsepower on regular fuel.

If you have any driving queries for Jason, send him a message at globedrive@globeandmail.com or contact him through Twitter: @JasonTchir

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