Regular vs Premium MPG report.

[bilworth]

Greetings,
I drive a 2014 5.0 R-Spec. I did a little test to see if using regular would adversly affect MPG. I live in the Colorado Springs area so my driving has been in higher elevations (between 6000 and 7000 feet). I don't know if that makes a difference or not, I thought I should mention it though. And, due to the altitude, the octane ratings are lower here than they are at sea level. I checked the milage with each fill up the old fasioned way and didn't use the car's MPG readout. I ignored the first tank full when computing MPG after changing to allow the OBDII to adjust. I drove 1,117 on premium and averaged 21.55 MPG. Then I changed over to regular and drove 893 miles and averaged 21.80 MPG. The slight difference might have been due to variations in traffic conditions. But, as you can see, regular seems to yield about the same as premium.

 

[Dawg]

I have always used premium in mine. I think I may gain some things like smoother idle and hopefully lower maintenance costs in the long run. I agree with you that premium probably does not improve mpg vs regular.

 

[T_D_Diggs]

I saw the opposite. I averaged about 28mpg on premium and about 25mpg on regular. The car seemed to have faintly better response on premium.
This was on a 3600km round trip of mainly (95%) highway driving. With cruise control set at 120kph. I do this round trip once a month. And have had nearly identical results multiple times.

 

[T_D_Diggs]

Also Hyundai themselves claim I believe about an 8hp difference between the two fuels. Though I don't know how valid that is.

 

[AgentSmith007]

The car seemed to have faintly better response on premium.

That's what I noticed too with my 4.6 when I switched to premium.

 

[LandoAWD]

Also Hyundai themselves claim I believe about an 8hp difference between the two fuels. Though I don't know how valid that is.

The ECU scales to knock threshold so better gas = less knock = more power. It's also why you can run 87 and not grenade.

 

[craigt]

Also Hyundai themselves claim I believe about an 8hp difference between the two fuels. Though I don't know how valid that is.

True for the 5.0. Not true for the 3.8

According to Hyundai, the 3.8 gets its fully rated power out of regular fuel.

 

[AgentSmith007]

True for the 5.0. Not true for the 3.8

And it is true for 4.6.
378 - regular
387 - premium

 

[craigt]

But that's 9hp.

 

[4104]

1000 miles on my G2 3.8 regular gas. 22 MPH city, 30.1 MPH highway.

I am happy...John

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[bilworth]

I saw the opposite. I averaged about 28mpg on premium and about 25mpg on regular. The car seemed to have faintly better response on premium.
This was on a 3600km round trip of mainly (95%) highway driving. With cruise control set at 120kph. I do this round trip once a month. And have had nearly identical results multiple times.

What elevation? Were you close to sea level? If so, I wonder if elevation might be playing a role. Possibly the ECU has dialed back the advance curve with both regular and premium at this elevation due to the lower octanes.

 

[svensk]

Driving at altitude 6000' + you REALLY need to be using premium on a car that's designed to run on it. Of course the ECU pulls timing based on knock, but a DI engine especially at that high up...

 

[T_D_Diggs]

The trip starts at 1100ft in the starting city. Up to 4500 ft then back down to about 1200ft.

 

[T_D_Diggs]

True for the 5.0. Not true for the 3.8

According to Hyundai, the 3.8 gets its fully rated power out of regular fuel.

Yes sorry my bad. I have the 5.0 so I automatically default to it as my reference.

 

[bilworth]

Driving at altitude 6000' + you REALLY need to be using premium on a car that's designed to run on it. Of course the ECU pulls timing based on knock, but a DI engine especially at that high up...

Hi there,

Why would lower internal pressure as a result of lower atmospheric pressure dictate a need for premium? I don't doubt you, I just want to understand.

Thanks,
Bill

 

[bilworth]

The trip starts at 1100ft in the starting city. Up to 4500 ft then back down to about 1200ft.

Well there goes my theory.

 

[kn5owa]

The price at Costco isn't that much more for premium - even a very small increase in mpg pays the difference. I use Mobil one following the same logic (and for other reasons).

But, it's nice to know that nothing blows up if you put regular in.

 

[427435]

Hi there,

Why would lower internal pressure as a result of lower atmospheric pressure dictate a need for premium? I don't doubt you, I just want to understand.

Thanks,
Bill

It doesn't. At higher altitudes, there is less air that is drawn into the cylinder and the peak pressure is less. Thus less octane is needed to prevent detonation.

 

[bilworth]

It doesn't. At higher altitudes, there is less air that is drawn into the cylinder and the peak pressure is less. Thus less octane is needed to prevent detonation.

That's the way I understand it. But Svensk seemed to imply there was something different with a DI engine. My knowledge of the internal combustion engine is a bit dated.

 

[bilworth]

Update: I have driven an additional 850 miles on regular and the mpg is remaining within a fraction of what I got on premium. However, many others have reported a drop in mpg when burning regular gas. I think I know why I am not seeing what others have reported.

If there are any engine techs out there, I would love to get your opinion regarding this theory.

Here's what I think is happening:

The atmospheric pressure at this altitude (7000 ft.) is usually around 11.34 psi as opposed to 14.7 psi at sea level. The compression ratio of this engine is 11.5:1. So, at sea level, the compression-stroke cylinder pressure in this engine would be approximately 169.05 psi (11.5 x 14.7). However, at this altitude, it would be approximately 130.41 psi (11.5 x 11.34).

The reason one would normally experience a degradation in engine performance/efficiency using regular gas in this engine is due to the ECU retarding the timing to prevent detonation. Detonation occurs when the fuel-air mixture in the cylinder explodes abruptly rather than producing a smooth burn. Higher octane fuel slows the burn rate to prevent detonation so the timing is not adjusted by the ECU. Detonation is a function of the pressure within the cylinder. At this elevation, the lower atmospheric pressure results in a lower compression-stroke cylinder pressure (130.41 psi) which is not sufficient to cause detonation using regular gas. Therefore the timing is not retarded by the ECU and the mpg is not effected. So, I believe I can burn regular and save money without suffering any penalty.....except the corresponding reduction in power the comes with the high elevation. I am driving a 340 HP sedan instead a 429 HP sedan....sigh.

What say you?

Thanks for any input.

Bill