[Gate Keeper]

Originally Posted by SD1too

Phil; the crucial point is that you zeroed the display when already driving at a constant 55 mph, crucial because:

• This speed is known to return the lowest fuel consumption.
• There was no acceleration involved, which lowers mpg.
• There were no cold starts or idling involved which lower mpg to a significant degree.

All these factors are part of "normal" driving and so need to be included in the measurement to ensure that the result is a realistic average.

You don't say whether you zeroed the display whilst already driving in the city. If you did, then 27 mpg is not a valid figure either. It's a short term "snapshot" in specific driving conditions.

To be able to quote a figure for overall, long term fuel consumption, the trip computer must be zeroed before the first cold start then left alone until at least an entire tank full of petrol has been consumed. Alternatively, use the brim-to-brim method under the same conditions. I'm afraid that I can guarantee that the result will not be 42 mpg!

Simon

I did write “CC was not in use”