syswei

On fueleconomy.gov, the S550 4matic coupe is listed as 16 city / 24 highway, the 4matic sedan 16/26. The S550 4matic sedan appears to have the same engine (someone correct me if I'm wrong), same 7G tranny (in the U.S.), is slightly heavier, and is taller. I don't know the drag coefficients, but to my eye the sedan looks less aerodynamic. So why would it get better highway mpg?

Could MB be fibbing?

dje.s550

I dont know the answer to your question. However my S550 sedan wouldn't get 26mpg unless I threw it off a cliff. The absolute best I ever saw was 24.5 and that was for a very short drive being extra conservative. General the average on highway is 21 to 23 at the very top. It's a pig.

MTrauman

Attached Please find my best MPG on a recent long 2000 mile trip. Normally mixed highway/city my mileage does not touch my reported 25.2 mpg. With that said, the picture is the best MPG I have been able to pull off on my '14 S550 4matic.

Cptdenny

Oink oink, I've seen 28 mpg @ 70 mph for about 30 miles on a 170 mi trip.
I usually average 25.5 mpg to 26.5 mpg @ 70 mph on the same road for the same distance here in South FL with 20" run flats.

Now for my secret: when using premium 93 octane fuel I only get 23-24 mpg under the same above conditions with my 2014 S550.
Experimenting with regular 87 octane fuel I get the better fuel economy.

This is why: the higher the octane the less BTUs in a gallon of gas.
If more octane is added to the volume of gas, some gas needs to be reduced to make room for the octane additive. Octane does nothing for fuel economy.

Octane is what keeps our engine from detonation or pinging. Since I haven't experienced any pinging or detonation and don't drag race my S550 for competition, I haven't found any loss of power or performance to date.

I started with premium like most of us do. Then mixing premium with mid grade, then just mid grade, then premium with regular, then just regular. My fuel economy continued to improve with no ill effect.

Experiment at your own risk. I'd start with a near empty tank, fill half with regular fuel and drive. If you have a hint of pinging then fill the remaining volume with premium fuel. If you have no I'll effect continue to use regular but monitor what you hear & feel. Start first with a baseline what premium gets you for fuel economy. Then see if there's improvment using the less expensive fuel.

With fuel prices dropping I've noticed premium prices falling slower than regular gas. It use to be a 20¢ to 30¢ price difference between regular gas & premium gas. I've seen locally as much as 80¢ a gallon difference between regular gas & premium gas.

syswei

I had read that 'modern' engines can use regular gas to no ill effect, so I switched to regular-only maybe 2 years ago. Nary a ping or any other issue evident.

EDIT: ...on my 2008.

dje.s550

I have had naturally aspirated engines before that stated they required premium and I used regular without any issues. However in my S550 I have been using premium (91 is best we can get here in CA) since new. I am worried that with the turbos it might have a problem or blow something up under boost pressure.

Any thoughts on this? I know in the Ford Ecoboost engines which are also twin turbo you can use regular with some "loss of performance" but the power difference can never actually be felt driving one. So I wonder if that is the same in the MB or if we would risk some failure under boost by using 87 octane.

Any comments on this?

Wolfman

Not sure about how the engine reacts, but I can tell you that MB would be happy to use that info as grounds for voiding a warranty in case of an engine problem/failure.

That said, these engines are designed to handle various grades of fuel around the world as many countries aren't as strict in controlling the fuel grades on pumps.

US fuel is not that great either with the ethanol supplement that varies by state.

Cptdenny

Some of your answers to your question can be found here:
http://www.edmunds.com/fuel-economy/...g-premium.html

I noticed that our cars "require" premium fuel so I'm taking a risk running on regular fuel. I may have to rethink my choice of fuels even though my fuel economy will drop using premium fuel. Oh well at least I know in a pinch the S550 will run on regular.

Cptdenny

Here's a question.
Since difference in regular fuel prices to premium fuel locally can be 80¢ a gallon, would adding octane booster to a regular gas fill up make better money sense?

Mitch Alsup

I can get 24 MPG in my S600!

It must be the way you drive.

Mitch Alsup

Both of the above are true, but why they are true is counter intuitive.

Lower octane will create greater pressure in the cylinders. As long as this greater pressure occurs after TDC the engine makes more power.

dje.s550

Might be California fuel as well. I know we have a different mix but not sure what is exactly different about it.