Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

89 vs 91 octane

I own a 2003 Mercedes 320CLK with only 17K miles. The owner’s manual calls for 91 octane, but I get 2 MPG better when I use 89 (25 vs 23)with no apparent ill effects.

City driving only, usually less than 5 miles one way - no jackrabbit starts - one driver only. Any ideas?

I would suggest that your additional mileage is due to measurement errors, including driving conditions. The difference between the two will have a minor effect on mileage. It will usually be too small to accurately measure.

One would expect that if you are using lower octane than recommended for the car, it would result in no change or a slight reduction in mileage.

I agree with Mr. Meehan. There’s no resason why it would get better mileage with lower than recommended octane fuel. If anything you should get slightly lower mileage. There’s also the slight possibility that the 91 octane you bought really wasn’t 91 octane. It’s a longshot, but not out of the realm of possibility.

I also agree with the other posters, but I’ve also seen situations in which the higher octane gas have higher percentages of ethanol.

The mileage you are getting is very good considering the type of driving and the lenght of your trips.

Wow, let me know when you want to sell it… 5-6 years and only 17K!

Oldschool has a good point. Some refineries do use more ethanol in premium gasolines. Ethanol may have a higher octane rating, but it does not burn with as high of a BTU.

They are trying to force us to use more ethanol in the Midwest where I live. I noticed my truck’s accelleration diminished when I used the higher octane, higher ethanol percentage (and ironically cheaper) premium gas. I also noticed about a 2-3 mpg drop with a 15% blend of ethanol.

I have heard people say they have seen as much as a 25% drop in MPG with E85 too.

Here’s what I know about octane and cars:

Higher performance (as I would guess with a Mercedes engine) will often have higher compression. Lower octane gas will explode prematurely and could cause significant engine damage. So much of newer vehicles are aluminum and pistons have been for many, many years. Aluminum, of course, melts at a pretty low temperature.

I have a set of pistons from a 400 Chevy that look like someone took a torch to them. These came from a truck built in the early 80s when gas went from 107 octane to a high of about 94.

If this is the case, you will hear a definite crackling noise from the engine when you accellerate. Of course, you have a nice, nice car so you may never hear it with all the sound proofing. I also wonder if the 2 octane difference would cause much of problem.

I hope this helps.

“Wow, let me know when you want to sell it… 5-6 years and only 17K!”

Why would you want a car that very likely has a severely sludged engine? When a car is driven in this manner–urban driving, 5 miles or less at a time, less than 3,500 miles per year–unless the oil is changed at least every 3 months, that engine is probably in pretty bad shape by now, along with the exhaust system.

The way that this car is operated is a textbook example of Severe Service, and unless the owner has been careful to maintain it in accordance with the manufacturer’s Severe Service maintenance schedule, I would only want the car if it was free.

I agree with the others that a difference of 2 MPG is likely either an error in measuring or is related to factors other than the octane rating. Even if there were no errors in measurement, there are so many factors that affect fuel economy that you can’t possibly know that the gas was the factor in the change in fuel economy.

I have two points I would like you to consider:

  1. If your Mercedes owner’s manual recommends 91 octane, but says 89 octane should be fine, you can use 89 octane without doing any real harm. However, if the owner’s manual says to only use 91 octane, you could damage your engine with the cheaper gas.

  2. Your Mercedes is an expensive vehicle. It is expensive to own, expensive to operate, and expensive to repair. So why be cheap about the gas? If you want to put cheap gas in it, you should trade it in for a cheap car that is designed for cheap gas. This car was made to perform best with expensive gas. So if you don’t want to spring for the expensive gas, you should sell it to someone who will take better care of it.

Thank you all for your thoughts.

  1. I can pretty much guarantee that errors in measurements have not been made. In addition to the available instantaneos computer readouts, I also check miles vs gallons purchased and have found it to be accurate.

  2. The owner’s manual does not say 89 is OK, but the local Mercedes people say it is if there is no evidence of knock/ping. I’ve tried heavy acceleration a couple of times to see if a ping would show up, but it has not.

  3. Driving conditions don’t vary much. Live in SC, lots of hot Wx but not that much cold.

4, Gas is not always purchased from the same station, but results haven’t varied, so I don’t suspect that I’m not really getting 91 octane.

  1. I too thought that it was unusual for the lower octane to yield better mileage -hence the question to begin with.

  2. Don’t worry VDCdriver - I don’t plan to sell or give it away. At my age it’ll probably be the last car I’ll drive.

89 octane is not approved for your engine. 91 octane is required for you engine. The owners manual was written with information provided by the factory engineers. If they say 91, and the local “Mercedes People” say different I certainly know which way I would lean. Seriously, there’s not that much difference in cost, do what the factory manual mandates.
3000 mile oil changes on the Mobile 1 is overkill,even in those tough conditions.
But wait, if he’s changing it once a year(Which Mercedes recommends if you don’t put on many miles) that would only be 3500 miles. So I would seriously doubt the Mobil 1 sludges at 3500 miles.


No apparent ill effects? When your car needs a new headgasket at 75-100k miles, that’s when you’ll realize that you should have been using the right octane. For me, if my car calls for 91 octane, I’ll use 93.

Remember that your car has a “knock detector” - this is NOT a knock preventer. Once it hears knock (which damages your engine obviously), then it will retard the timing to get rid of the knock. And although one ping won’t kill your engine, a few pings per mile over 75k miles can certainly doom your headgasket.

Just remember that a knock sensor is a REACTIVE device. Better than nothing, but unless you have a scan tool (or Mercedes has an unusually complete gauge package), you don’t know when it has heard some knock.

On the other hand, you are also correct that the lowest possible octane (without knock) will theoretically get you better gas mileage due to the slightly higher volatility (and therefore more complete combustion), but I suspect it will be more like 0.2 MPG than 2 MPG.

And based on the few miles that you use this car, it’s a very cheap investment to put 93 octane in there.

By the way, if you only drive 5 miles at a time, this vehicle is never getting up to temperature. That’s bad. You should take the car for a long drive sometimes, just to get things hot and burn some nasty gunk out of the oil. Your type of driving counts as “severe” and you should increase your oil change frequency- not because the oil viscosity is bad, but because of the stuff that builds up in your oil that won’t get burnt off when the engine is cold.

Has it occured to you that the local Mercedes people look forward to making money on unnecessary repairs? It is in their interest to give you advice that will lead to damage to your engine. When you are penny wise and pound foolish, they make money.

I guess you had your mind made up already though. It is a shame.