MPG ratings - a scam?

[vi edit]

OP doesn't understand that any appreciable amount of boost requires higher octane. That dart has a turbo, so it needs premium fuel.

Cruze, Ecoboost from Ford, and Hyundai's recent turbo engines are all mid-grade fuel. None of them say premium needed for their stated MPG's.

 

[phucheneh]

OP doesn't understand that any appreciable amount of boost requires higher octane. That dart has a turbo, so it needs premium fuel.

This is totally dependent upon compression ratio and boost level. There are plenty of <9:1 engines with low pressure turbos that do not require premium gas.

Ideally, it should be a fairly even trade given similar power. The engine requiring the premium fuel is going to make its power more efficiently and should get better mileage. But that's obviously not always gonna be the case.

 

[leper84]

But to lower octane requirement, would you not still have to lower compression ratio? The lesser amount of air being brought into the cylinder would still correlate to a sea level engine that is simply running at a different speed/load, right? I can't see this any other way in my head, but I have absolutely no idea if I'm actually right.

Your effective compression ratio does lower at higher altitudes. A motor with a static compression ratio of 9.5:1 would have an effective ratio of 8.5:1 at 5000 feet above sea level.

Detonation really only occurs at high rpms or high load/low rpm situations. Either of those cases you are going to have a lower overall pressure in the combustion chamber due to the less dense air at WOT. Less pressure means you can use a fuel less resistant to pressure.

Whether you're in Austin or Colorado Springs if you are having detonation at part throttle cruising you've got bigger issues than altitude or 85 octane.

 

[wirednuts]

i think midgrade can sometimes be a good buy, because the regular stuff so often is so crappy. i can tell just by the way my truck runs, sometimes i swear i bought a gallon of water with my 20 gallons of gas. but i almost always buy the cheapest gas. my truck gets 15-20mpg and aint nothing going to change that.

 

[LTC8K6]

Cruze, Ecoboost from Ford, and Hyundai's recent turbo engines are all mid-grade fuel. None of them say premium needed for their stated MPG's.

Not sure about the other two, but Ford is pretty clear that premium gives the best "performance" in certain engines.

The Dart was a flop in 2012. Pretty dismal sales.

 

[vi edit]

Not sure about the other two, but Ford is pretty clear that premium gives the best "performance" in certain engines.

The Dart was a flop in 2012. Pretty dismal sales.

They certainly point out that power is different between premium and regular but I couldn't find anything that stated that MPG is different between fuel grades.

 

[LTC8K6]

It helps sales if you can say it runs on 87 octane, and then just put in the manual that it "performs better" on premium. I think that's all we are seeing there. Fear that a premium requirement will affect sales.

I can see the other side of the coin as well. If it got 1 more MPG on 91 octane, then it might help sales to say so. I really think the price of premium would outweigh that in people's minds, though.

We would need the EPA to test both and see, but the EPA apparently uses fuel we can't even get.

In my mind, "performs better" is a very good thing...

 

[jagec]

My main grip via the OP is that are the manufacturers comparing apples to apples in terms of mileage; when they are using a different set of fuel to generate such numbers.

I have never noticed any other MPG comparison stating the Premium must be used.

They are telling you the mileage that the car achieves using its intended fuel. You don't call it a "scam" when flex-fuel cars quote mileage figures on regular fuel, instead of the lower E85 numbers. So why is this any different?

The ability for high-compression or boosted engines to run AT ALL on lower-octane fuels is a relatively new development, but just because they CAN doesn't mean that anyone should.

 

[jagec]

i think midgrade can sometimes be a good buy, because the regular stuff so often is so crappy. i can tell just by the way my truck runs, sometimes i swear i bought a gallon of water with my 20 gallons of gas. but i almost always buy the cheapest gas. my truck gets 15-20mpg and aint nothing going to change that.

Mid-grade is 50% premium, 50% regular, mixed at the pump. There is no separate tank for "mid-grade".

 

[LTC8K6]

Mid-grade is 50% premium, 50% regular, mixed at the pump. There is no separate tank for "mid-grade".

Yep, that's why some stations sell only 87 and 91. They saved money by not buying blending pumps.