Why is MPG so much better on 97 super unleaded?

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Psilonaught

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Jul 22, 2017
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159
Location
Near Saffron Walden
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W212 E63
Unfortunately, my village garage doesn't do 97, so for the last couple of weeks i've been running on 95.

I drained the tank today and filled up with 97, and then reset the MPG couter. In eco, on b roads at around 50mph the average i was getting around 31mpg. This is amazing and far higher than with 95.

What is the reason?
 
You should only notice an improvement if your engine can benefit from higher octane fuel. - Usually that means cars with high boost that are prone to detonation at WOT.
 
You should only notice an improvement if your engine can benefit from higher octane fuel. - Usually that means cars with high boost that are prone to detonation at WOT.

My AMG is meant to run on 99 :)
 
If the engine is designed to run on 98 or 99, there will be a slight drop in performance and efficiency when running on say 95.

If your car is designed to run on 95, I doubt you will notice any improvement using anything more.

I've certainly noticed running a car on a lesser fuel than it was designed for, but not noticed vice versa.
 
Psilonaught said:
Unfortunately, my village garage doesn't do 97, so for the last couple of weeks i've been running on 95. I drained the tank today and filled up with 97, and then reset the MPG couter. In eco, on b roads at around 50mph the average i was getting around 31mpg. This is amazing and far higher than with 95. What is the reason?


Check your petrol flap/manual your car needs 98, 95 should only be used as a temporary fix if no 98 is available.
 
Check your petrol flap/manual your car needs 98, 95 should only be used as a temporary fix if no 98 is available.

Never seen 99? Use 98 when available but mostly and by that I would say 60-70% of fuel is 95. No issues what so ever that I can note
 
The petrol flap on my m156 car states 98 Ron and the manual states always use 98.
 
With higher octane fuel which by definition less given to pre-ignition, the ignition timing can be advanced further producing more output (and economy). With lower octane fuel the knock sensor will pick up any pre-ignition and retard the ignition timing via the ECU. The effect is meaningful at the margin and really it is much better to run engines on the correct octane (you will really notice it on older engines without a knock sensor tuned for 99+ octane, they will pink and get hot and do themselves no good at all).
 
Deleted.
 
What's this 'Eco' mode you speak of?
 
What's this 'Eco' mode you speak of?
About 5 or 6 years ago, one of the COMAND updates introduced an 'ECO' routing option.

One of my work colleagues asked, "What does that do? Direct you to the nearest railway station?"

:D :D :D
 
Our M156 (NA) and M157 (TT) engined cars have a sticker inside the fuel filler flap which states that minimum 98 RON super unleaded should be used, whereas our previous generation supercharged AMG simply stated 98 RON?
 
But 98octane is more expensive than 95 so costs the same per mile.
 
Never seen 99? Use 98 when available but mostly and by that I would say 60-70% of fuel is 95. No issues what so ever that I can note



Costco have opened their own petrol station and their premium unleaded is 99


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But 98octane is more expensive than 95 so costs the same per mile.

If you use 95 when you are meant to be using 98 or more, you will get worse economy though.

I've never done the maths though although when I bought an AMG, MPG was not a consideration for me so I ensured I used 99RON as I want it to run the best it can and how the manufacturer intended.
 
I've just been chatting to the other guy(not John) at BFT(looking at 660bhp and 1090ft/whatever!) about this and he says for optimum economy and performance use shell V power or Tesco's good stuff! He was saying it's a must really.
My flap states 98 ron.

On this theme, I'm heading back up to Loch Ness again in my car, do any of the Scottish contingent know if Gleaner fuels do a higher octane fuel? As I'll need to fill up but won't really be going near Inverness. Or if there is somewhere on the A9 Dalwhinnie area? Ta :D
 

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