MPG increases after a long run

obi_waynne

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Deal, Kent UK
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A3 1.4 TFSI 150 COD
Why does the MPG returned by my car go up significantly after a long motoway run?

It's noticeably up on what we usually get for the same journeys, 45 mpg is now 53 mpg. The only difference being the fact I've done 800 motorway miles last week.

Does this mean the car is getting clogged up on my usual drives? Is the fuel I'm using partly to blame? Do engines change the fuelling/timing/boost based on what the car does over the last few hundred miles?

What is the best way to keep the car in this super economical mode?
 
I get this on my Golf and put it down to the DPF having to regenerate more because of the short runs it normally does. After a nice long trip my mpg improves dramatically. There's a 20 mile round trip we do 3 times a week and the mpg can be as low as 38mpg when dpf is regenerating but goes to its more regular reading 54-60 mpg after a good clear out from a long trip.
 
Diesels are whores in that respect. They are very economical in low speed use, yet the best way to sustain good economy is to drive them hard !!
 
Is this momentary MPG or average over time? If it is an average, then your avg MPG will be up after running at highway constant speeds. If it's momentary MPG, I don't know why it'd be up after a trip on the highway.
 
I think we're talking about average here. Then again, momentary is average too, just over a much much much shorter time frame :)

Diesel engines advantages over petrol ones dimish with increasing speed in my experience.

Sustained load speed driving is very good at clearing out diesel accumulated junk in the exhaust and turbocharger. This would explain the claims made above.

Mine has no DPF and it's startling how much clag accumulates in low speed town driving. First visit to the red line after lots of town work results in an impressive grey cloud of soot etc.
 
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Is this momentary MPG or average over time? If it is an average, then your avg MPG will be up after running at highway constant speeds. If it's momentary MPG, I don't know why it'd be up after a trip on the highway.
Were talking the next 400 miles of urban driving after a 800 mile motorway run compared to the 400 mile urban driving average before the long run.
 

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