MPG problems

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I have bought a 58 plate Honda Accord 2.2 diesel (145ps version).

Quote combined MPG: 48
Actual I am achieving: 39 - This is 90% motorway mileage.

I have done 600 miles over the last week, mainly motorway, resetting the MPG counter when I bought the car and after 300 miles. Something really doesn't seem right, I averaged 47mpg in my D5 Volvo that is quoted as 42mpg combined!

Any ideas? Or does anyone have a 2.2 Accord that can share their real world MPG?

 

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Serviced by the garage when I picked it up. Tyres are inflated to exactly what the book says.

 

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I haven't worked it out exactly at the pump, but I am already well into my second tank of fuel after 600 miles and put in over £60 when I filled up. so I can't be getting anywhere near what I did in the Volvo.

I am actually driving slower in the Honda, I certainly did for the second 300 miles. On the way back home on the A1 I drove well within the speed limit all the way to see if it helped.

 

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Average speed on combined cycle test: 38mph
Top speed on combined cycle test: 75mph (for a matter of seconds)

The majority of the combined cycle is spent sat at about 45mph or so. Drive around constantly at 45-50mph and you'll probably get 48mpg easily.

Interesting. What about extra urban where it says I should be getting 50 something?