On my 91 Eldorado I discovered I get better milleage with lower octane fuel when compared to higher octane (91, in this area). This doesn't make any sense. Especially since 91 Octane is the recommendation.

My only guess is perhaps the fuel was crap. But I doubt that because it's a reputable place and I've heard nothing bad of their gas.


hmmm....Odd...We just had an earthquake. heh. Felt like the damn house was dancing.

Somewhat possible if the premium you were buying had alcohol in it. Lots of premium fuel has some alcohol to raise the octane rating of the fuel. It takes twice as much alcohol by volume to make the air/fuel ratio correct compared to gasoline so if the fuel you were buying was 10% alcohol then the mixture would be 5% lean and the closed loop control would react and add more fuel thus causing slightly poorer fuel economy.

Honestly, it's very difficult to make real world fuel economy comparisons, especially based on only one tank of fuel. Driving conditions, weather, wind, etcetera all make far more difference to the fuel economy. Only on a direct comparison or over many tanks of fuel will a small difference in fuel economy show up.

There's certainly nothing (other than the alcohol explaination) from an engineering standpoint that would substantiate or explain getting better fuel economy on regular compared to premium gasoline in an engine (like yours) that does not have a knock sensor that would be retarding the spark due to detonation or something. No rational at all really to support getting better fuel economy on regular. Must have been alcohol or the driving conditions for that tank of fuel.

I'm going to have to find out if that station uses Alcohal in their fuel. Like you said, I wouldn't be surprised if they did.

I know you're right on how driving conditions and all that make a real world MPG rating near impossable. The only reason I found it odd is because there was a 4 MPG difference(18 on high octane and 22 on low octane.) My driving habits stayed the same, too.

ive been using 93 octane for over a year since i got my caddy. for the most part i use the same filling station as well. my mpg will range from just under 16 to just over 20. my driving habits stay the same, i drive the lac like it was meant to be driven.

the reason i say this is that you thought the 18-22 was a huge breakthrough. i run 93 constantly and get mpg between 16-20...theres really no telling what it will be.

I'm going to have to find out if that station uses Alcohal in their fuel. Like you said, I wouldn't be surprised if they did.

I know you're right on how driving conditions and all that make a real world MPG rating near impossable. The only reason I found it odd is because there was a 4 MPG difference(18 on high octane and 22 on low octane.) My driving habits stayed the same, too.

It is very simple to test for alcohol in gasoline. Alcohol will mix with gasoline or water but water will not mix with gasoline and alcohol has a greater affinity for water than gasoline.

Put an equal amount of water and gasoline in a graduated cylinder and shake up thoroughly. Note the dividing point between the water and the gasoline before and after mixing. If the gasoline decreases then the gasoline had alcohol in it. For instance, if you put 100ml of gasoline into a graduated cylinder and added 100ml of water and then shook them up and let it settle out and the water is now at 108ml and the gas is at 92ml the fuel was 8 percent alcohol.

chuckdobbins, Thanks for the information on that. appearently nothing too interesting is happening with my mileage. :-)

KC, I'm going to have to test that out. I am curious about it now. Thanks for the excellent explanations thus far.

Some thoughts to ponder on:

87 octane and 93 octane have almost the same BTU's per gallon. In a nutshell, there is no more explosive "power" in 93 octane than 87.

87 octane is easier to ignite (lower specific gravity)
87 octane burns faster than 93
87 takes less barrels of crude to make than 93 (cheaper)

with modern fast burn cyl heads, they can take advantage of the lower octane with the same timing requirments of the "old design" heads. So better power, better efficiency through technology.

So, YES, with NO other engine changes (or an engine that can adapt to octane levels by adj timing), you WILL get better mileage. Now, changing the timing to use the higher octane you MIGHT get better mileage.

Tom