Calling our car experts

[Abbey Marie]

Please help solve a debate between two friends.

Two questions:
1. Do some/all Mercedes need higher octane gas?
2. If they do, is it ok or even a smart idea to mix 1/2 Premium and 1/2 Regular to get a good result while saving money? I'm not sure why she doesn't use Plus, but she doesn't.

Thanks!

[tailfins]

Please help solve a debate between two friends.

Two questions:
1. Do some/all Mercedes need higher octane gas?
2. If they do, is it ok or even a smart idea to mix 1/2 Premium and 1/2 Regular to get a good result while saving money? I'm not sure why she doesn't use Plus, but she doesn't.

Thanks!

Many six cylinder luxury cars call for premium fuel, apparently all Mercedes Benz models do. However, the most modern ones have the ability to compensate by reducing engine power, allowing the use of regular unleaded if you're willing to accept lower power and/or gas milage. The purpose of higher octane is to prevent "knock", a form of misfiring that can damage the engine. Adding the two in the same tank does not mean you are mixing them, no more than dumping eggs and flour in a bowl means you are mixing those.

If your MB is 2010 or later, and you're OK with less engine power, you should be OK with regular unleaded. If your owners manual calls for 91 Octane, then mid-grade on the East coast meets that requirement.

[Abbey Marie]

Many six cylinder luxury cars call for premium fuel, apparently all Mercedes Benz models do. However, the most modern ones have the ability to compensate by reducing engine power, allowing the use of regular unleaded if you're willing to accept lower power and/or gas milage. The purpose of higher octane is to prevent "knock", a form of misfiring that can damage the engine. Adding the two in the same tank does not mean you are mixing them, no more than dumping eggs and flour in a bowl means you are mixing those.

Thanks for the info.

If they are not mixing in the tank, what exactly is happening in there? Can it be bad for the car?

[tailfins]

Thanks for the info.

If they are not mixing in the tank, what exactly is happening in there? Can it be bad for the car?

To some extent, they would "take turns". You would be running sometimes on 87 Octane, and sometimes on 93 Octane as you ran through different areas of the tank. If you insist on using regular unleaded, don't drive with a lead foot. My wife's car is a Japanese luxury model that calls for Premium. I only run regular in it if Premium is not available. I would worry about shortening the life of the engine with regular unleaded unless it's a lease vehicle, since you will turn it in long before worrying about slightly shortening the life of the engine.

[NightTrain]

The car will retard the timing when it detects that it's running a lower octane than it should, as Tailfins described above, which results in a little less power. I would imagine that the O2 sensor, exhaust sensor, and all the other engine monitoring sensors work hard (along with the computer) to adjust to the different burning characteristics of the fuel.

Components that work unusually hard will fail sooner.

It's certainly not worth rolling the dice over... if you're driving a highly engineered vehicle like a Mercedes, you should follow their guidelines.

Mercedes is not telling people to spend more money at the pump while driving their cars because it gives them thrills. It's necessary for the proper operation of the engine and if it didn't matter, they would definitely say so.

If you're out in the middle of nowhere on a trip and all that's available is 87, I wouldn't worry about using it. But deliberately putting lesser grade fuel in it regularly is not a good idea.

[aboutime]

Bottom line: If the Owner's Manual suggests, or recommends using Higher Octane fuels.

There must be a reason they (the manufacturers) said so. BUYER BEWARE should apply.

If you can afford the Automobile in question. Why should saving a few pennies per gallon even
be a question. Other than simply...wanting to know the differences, or why?

[Little-Acorn]

I'm trying to figure out what "different areas of the tank" means.

When you fill your tank half full of regular, then fill it the rest of the way with Premium, they two do mix. It's as simple as it sounds. And you wind up with gasoline that has more knock-suppression capability than regular, and less than premium.

Some trucks have two gas tanks (or more). On those, if you put regular in one tank and premium in the other, they don't mix, obviously. But that's the only situation I can think of where the two kinds of gas won't mix. And it is rare. I've only seen it in trucks, and only a few of those. I've never seen a car where they wouldn't mix, immediately and completely.

[tailfins]

I'm trying to figure out what "different areas of the tank" means.

When you fill your tank half full of regular, then fill it the rest of the way with Premium, they two do mix. It's as simple as it sounds. And you wind up with gasoline that has more knock-suppression capability than regular, and less than premium.

Some trucks have two gas tanks (or more). On those, if you put regular in one tank and premium in the other, they don't mix, obviously. But that's the only situation I can think of where the two kinds of gas won't mix. And it is rare. I've only seen it in trucks, and only a few of those. I've never seen a car where they wouldn't mix, immediately and completely.

Ask a tank truck driver if it's OK to add Premium to Regular and sell as mid-grade. He will tell you it's not allowed because the state inspector can take a sample and potentially get Regular, resulting in a huge fine. BLENDING fuel is not that simple.

[jafar00]

As I understand it, higher octane burns more efficiently and is cleaner for the engine anyway. My car is "suitable for" E10, but I put 95 in it because even though the price is higher per litre, it is more economical according to a bunch of tests done. 95 is best followed by 98 then E10.

[tailfins]

As I understand it, higher octane burns more efficiently and is cleaner for the engine anyway. My car is "suitable for" E10, but I put 95 in it because even though the price is higher per litre, it is more economical according to a bunch of tests done. 95 is best followed by 98 then E10.

I presume Australia uses Research Octane? The US uses the average between Research and Motor Octane. That's why in the Eastern Time Zone you will see 87 for regular unleaded, 89-91 for mid-grade, and 92-93 for premium. Because of the elevation lower octanes are sold in the some of Mountain Time Zone (Denver for example) where regular unleaded is 85 octane.