[JA-06]

Hello folks!

For some reason, 93 gas has become rare in my area so I’m stuck with 93 non top tier or 91 top tier ethanol free. I wanted to see if there was any difference between both. I had to do a round trip today, total 420 miles.
I will only use % of fuel tank since range announced by idrive is subject to driving habits.
Left the house with a 51% fuel tank, had to stop after 165miles because I only had 13% left.
Fueled up with 91, drove the 45 miles missing and went home. Total drove with 91 250 miles and still with 56%.

According to the app, 93 gave me 21.6 mpg while 91 had 24. Traffic was pretty much both way the same and speed as well (70mph). Car ran a lot smoother with 91 so I assume that top tier is more important than the actual octane being used.

What do you guys think?

[eelnoraa]

I think octane rating has nothing to do with mpg. It impact peak power if the car can take advantages of thr higher octane.

[AvGeek]

The "93" likely had 10% ethanol, which has a lower energy content than the pure gasoline (ethanol actually increases octane). So the "91" actually is more energy dense and pure than the 93, which makes sense on the mpg difference.

[X6ix GTA]

Originally Posted by BG X7

The "93" likely had 10% ethanol, which has a lower energy content than the pure gasoline (ethanol actually increases octane). So the "91" actually is more energy dense and pure than the 93, which makes sense on the mpg difference.

This ^^^^. I think it is the lack of ethanol in 91 that made the difference.

[BlkGS]

Originally Posted by BG X7

The "93" likely had 10% ethanol, which has a lower energy content than the pure gasoline (ethanol actually increases octane). So the "91" actually is more energy dense and pure than the 93, which makes sense on the mpg difference.

This will absolutely happen, but more ethanol let's you run more timing and make more.power.

[crackerjack15]

Originally Posted by BG X7

The "93" likely had 10% ethanol, which has a lower energy content than the pure gasoline (ethanol actually increases octane). So the "91" actually is more energy dense and pure than the 93, which makes sense on the mpg difference.

Exactly. Ethanol has ~76k btu's vs gasoline at ~114k btu. So e10 which is normal gas these days has maybe 112k btu. Stoichiometric for straight gas is 14.7:1 and for e10 it's 14.0 - 14.1:1

So basically gas with e10 will have to flow more and get less mpg.

I run E85 in my mustang. Equivalent to approx 105 octane. I get a dismal 12mpg most of the time. But she's laying down up to 1100whp depending on boost level.

[TurtleBoy]

I would say nothing can be concluded from the numbers. Unless you were driving the same route, the same way with the same weather there is no use trying to compare such a small sample size.

[AGONXF30]

Originally Posted by BG X7

The "93" likely had 10% ethanol, which has a lower energy content than the pure gasoline (ethanol actually increases octane). So the "91" actually is more energy dense and pure than the 93, which makes sense on the mpg difference.

I am not sure that he is actually getting Ethanol free gas! But if he says so! But I would say you are correct if the fuel has 10% ethanol may create a 5% impact on fuel economy since ethanol has less than half the energy density of gasoline. So combined with error, 91 vs 93 and the 10% ethanol we can see his 11 percent better gas mileage. The data set is too small to say if the overall increase in MPG will be this high, but I would expect at least a 7 percent increase over the long run.

The fuel with lower octanes have higher energy density is absolutely true. So if the car was designed for 87, it will get slightly better MPG with that gas and run equally as good, unless it has a modern ecu (like Mazda where they give you power ratings for 87 and 93) where the knock sensors can aid to adavce the timing, fueling delivery, etc. to produce more power in the absence of knock from the higher octane. I am sure BMW may do similar, but not advertised.

The X5 was designed for 89 and above octane, so I bet with 89 and normal driving one would see even better MPG, but slight reduced power. Heck BMWs power rating are most likely for 89 octane.

[Georgetheowl]

your test is completely inconclusive due to the small sample qty. In theory there is 3% difference on E10 vs pure gasoline, but who knows if the engine wouldn't run 3% more efficient on E10? Only a carefully controlled environment with multiple repeats will give you conclusive answers.

[sac02]

George and Turtleboy beat me to it.

I think your test is meaningless unless you have a MUCH larger sample size, or a MUCH more precise test environment.

Source: OEM engineer who does fuel economy and emissions testing (among other things).