My 1994 Mercedes uses premium gas–octane rating of 91. I gassed up a little while ago, and at the price of $2.51.9 (0.341 gallon, according to calculations) I noticed I had the wrong grade, and restarted the pump, and finished the $20 purchase with the correct grade–7.040 gallons at $19.14 (octane rating 87). Has anyone else blundered like this? I hope this won’t cause problems… :eek:
The computer should be able to compensate. You will get decreased performance / mileage.
Your story seems off. You say that your car takes 91 octane, you bought 0.341 gallons of the “wrong grade” and then finished by buying 7.040 gallons of the “correct” grade of 87 octane.
I assume that you meant that you bought 0.341 gallons of 87 octane and 7.040 gallons of 91 octane. If so, you bought a weighted average octane of 90.8. Added to the presumably correct octane gas in your tank, this difference in octane is entirely inconsequential to your car.
Thanx to both. 
Except that the correct grade had the octane rating of 91. ( Excuse me–I’ll have to check that…)
I’m a little confused by your numbers. Are you saying you filled up mostly with 87 octane or mostly with 91 octane? Because if you put just a little 87 octane in a tank that is mostly 91 octane, the tank overall is going to remain pretty close to 91 octane, which would be no biggie.
Anyway, the problem with using a fuel with too low of an octane rating for your engine is that the fuel will detonate just from the compression alone, before it reaches the point where the spark should set it off. This causes pinging and knocking, which you can usually hear while the engine is running. The early detonation of the fuel can also be rather damaging to the engine, as the fuel is starting to explode when the pistons and valves aren’t quite in the right position for it yet.
The thing is, most cars built since the 1980s or so have knock sensors in them, and can compensate for the low octane fuel. Your gas mileage may suffer, but you generally won’t cause any engine damage.
My 1980s era Cadillac recommends 91 octane or better, but it will run on anything down to 85 octane without damage. It gets its best mileage on 91 octane, but it gets its most economical mileage on 89 octane, since the mileage increase from 89 to 91 octane doesn’t equal the cost difference.
For the benefit of others coming into the thread, if you go the other way (put 91 octane in a car made for 87) you generally won’t do any harm, as all you are doing is just paying extra for fuel that won’t spontaneously combust from pressures that your engine never reaches anyway. While octane rating only says how much you can compress the fuel before it combusts and doesn’t say anything else about burn rate, energy content, etc., a lot of higher octane fuels tend to burn a bit slower, so the higher octane fuel might get worse mileage in your 87 octane car than the fuel it was designed for. We had an old Toyota that was designed for 89 octane. It would run on 87 but the mileage would drop. It would also run on 91, but again, the mileage would drop, though not as much as it did on 87.
Yes, 91, according to the Owner’s Manual. Most of the gas had that octane rating.
This.
I recently switched my 2003 BMW (185K miles and going strong) to regular gas. The manual says it wants 93 octane premium which I’ve given it since new. The consequences: Zero apparent change in performance, zero apparent change in fuel mileage, 25% reduction in fuel costs. Shoulda done it 10 years ago. :smack:
The OP should try a tank of pure 91 and then a tank or two of pure regular. Keep careful records of fuel mileage, performance, etc. If you don’t see any decline in mileage or performance that matters to you, keep burning regular.
The one thing that’s 99.44% guaranteed is that a 1994 car won’t be harmed by regular.
Less than 5% of your fill was 87 octane. Neither you nor your car will notice any difference. No harm done.
I personally haven’t made a mistake of that caliber for, oh, a couple of hours now.
The Perfect Master Speaks: What’s the difference between premium and regular gas?
Some cars require premium to run correctly. Usually, if you put in non-premium, you’ll know fairly quickly if that’s the case for yours. Mercedes Benz strongly recommends premium gasoline for their engines, so it’s probably not a good idea to do too much experimentation to find out if they are right. But in the case of the OP, the difference is probably de minimus.
I knew a guy who would fill up his company car where the rest of the truck fleet would. Once he put about half a tank of diesel in before he noticed he was at the wrong pump and diluted it with gasoline. This was long before EFI or anything else fancy but the car ran okay (albeit with a fair amount of smoke).
I’ve mentioned, elsewhere on this board, a cousin of mine who used to work at a boat fuel dock in Redondo Beach, CA. Once he had followed careless instructions of a boat owner who had mistakenly told him to fill the boat’s fuel tank with gasoline! It turned out it was supposed to take diesel; my cousin told him it would take hours to drain the gasoline from the tank, lest they both be blown sky-high when the guy started the engine. :eek:
I’m surprised at reading the gas grades in the US are substantially different to Australia. Here, the game starts at regular @ 91 octane, then premium is 95, and ultimate (and other marketing derivatives thereof) are at 98 octane. Of course, fuel is far more expensive and average altitude is very low. But I doubt either of these are factors in the difference.
So, why the difference?
As best I can tell, this is a difference in marketing. There are two different octane numbers in play: RON and MON. MON tends to be lower than RON: Wikipedia says 8-12 points, so let’s go with 10.
In the US, octane numbers are reported as the average of the two, but in Australia it’s the RON. Suppose we have RON=96 and MON=86 for some given grade. The the US, this would be reported as 91, but in Australia 96. So it’s not as far off as it sounds.
A very similar thing happened to my father-in-law (though, it was in Florida, so it wasn’t the same person / event). My understanding was that most boats of the size that my father-in-law had (40’) had diesel engines, but when my father-in-law bought the boat, he insisted on gasoline motors, since he was a mechanical engineer who specialized in gasoline motors (meaning that he could easily work on the engines himself).
Anyway, they pulled up to a fuel dock in Key West, and while my father-in-law told the guy running the pumps “gasoline”, either the guy didn’t hear him, or assumed he meant diesel (since that was the standard). My wife realized what was happening, halfway through the fill-up, leading to several hours of siphoning out the tank.
I had a friend who, after a couple of “near misses” like this, made a firm rule that he would always be present when fuel was pumped. The word “gasoline” can be understood as “fuel”, or possibly as something an ignorant diesel-powered boat owner might say.
Never store kerosene in a gasoline container … mistakes happen …
Are Mercedes really of such poor quality that they can’t tolerate a little of the wrong fuel? If I was paying that much for a car, I’d insist on something more robust than that.
It’s not a matter of quality at all. It’s a matter of what the engine is designed for.
It’s not so simple: I have repairs pending on the car’s engine and I don’t want to complicate matters.