I have a 2004 Mondeo 2.0 TDCI. It's a great car and has done about 127,000 miles.

It had a service recently and had a new timing pulley and belt as well as four new tyres in the last month or so.

In a typical commuting week I used to get about 57-57.8mpg out of it. After some heavy driving the mpg dropped to just below 56 and now, a few weeks on, it's not really going above 56mpg. In fact it often drops to 55.8. My commute hasn't changed, and if anything I'm less heavy on the accelerator.

Any ideas as to why it's dropping?

Fuel, tire pressures?

There could be loads of possibilities.

As steve said, different fuel or tyre pressures.

Due to the weather getting colder there will be some loss in mpg compared to the summer, but exactly how much im not sure.

The heavy driving you did may have cleaned out a build up in carbon somewhere in the system, so the engine is now running cleaner allowing more fuel or air through, or even cleaned a sensor that may have been reading wrongly.

There could be a split in one of the hoses (usualy the intercooler to egr hose) or the egr playing up. Although this is more than just a change in mpg, there may be noises light whistles or gas escaping and often rough idling.

When you say your less heavy with the accelerator, what do you mean, are you not pushing it as hard, or are you changing up the gears sooner. From what I have read in several forums, it is more economical to drive around 1800-2100 revs, rather than lower down in the rev range (1200-1600), so for example stay in 3rd when doing 30mph.

There will be other reasons, Those above were just a few possibilities.

Are your mpg calculations from the dash or do you do manual calculations, ie. litres used to miles driven? The cars comp is known to be out by up to 10mpg in some cases.

I know these differences are only going to have a minimal effect, but several small things can add up to a larger effect on mpg.

Id suggest first giving the MAF and MAP sensor a clean, possibly reset the ECU. Stick with the same petrol station for the time being to rule out different fuel, reset the mileometer and mpg on the dash every fill up.

When you fill up make a note of how many litres you put in and how many miles you have done and use on of the many online calculators to work out actual mpg.

P.s.

The pulley belt wont be for timing as mk3s are chain.

57mpg on commute I get 48 going work and back

Only get 53mpg on motorways

all mine is nearly urban im lucky to average 38mpg lol

my driving is mainly a couple of miles at a time trips so never gets warm, most of the time my manually calculated mpg is around 39, the dash says 42-43. If I do a long run on the motorway it will go up to the high 40s.

I always get lower MPG this time of year, because it's colder. Plus, My daughter started school so I'm doing two daily school runs of about two miles each way, and that doesn't help. I think I'm at about 48MPG now, whereas back in the summer it was about 53MPG.