I have a 54 Mondeo TDCI. I have had for a while my mpg about 57-58 and am getting about 620-650 miles out of the tank.

However, my mpg is now up to 59.4 and I'm getting about 580 miles - same journeys, same style of driving but quite a big difference. Anyone know why?

Are you working your mpg from the dash display or manual calculations?

From the dash display

The dash display is more a rough guide than an actual result, if you want accurate readings you need the number of miles, the number of fuels and a calculator.

However, given the time of year, although the MPG may appear higher, the cold will affect the efficiency, but as well as that, the road conditions affect the resistance.

If the MPG is as high as it looks, then the engine is driving efficiently, however, if the roads you are driving on have increased resistance, all that benefit is lost as your moving the car, so then you get more MPG and less miles.

Check your tyres, tracking, air pressure as a start point, make sure they are at an optimum and then monitor from there.Also do the MPG calculations manually and compare to what you see on the dash. The Dash can read differently also as the dense cold air can fool the car into thinking its using less air, so it injects less fuel to compensate, thus the car sees its going further using less juice when in fact it isnt.

I find the MPG drops quite a lot when the weather is cold. I get about 53MPG in summer, down to 47 in winter.

But yeah, pretty much ignore the trip computer. I'm pretty sure it just pulls numbers out of a magic hat.

As above, do manual calculations rather than relying on dash.

Alot of mondeo drivers have found their mpg can be out by as much as 10-12mpg on the dash compared to manual calculations.

Usually the mpg goes down in cold weather, so your going up is a little strange, it would be nice if it has though :)