My wife's Toyota Yaris diesel (2011 model) has been averaging around 60mpg since we bought it at 40,000 miles/two years ago. The car has now done nearly 70,000 miles and, over the past six months, the mpg has started falling steadily to around 55mpg.

I realise that mpg tends to drop over winter, but it did not change much in the previous winter and has not picked up in the recent warmer weather. Do you think it is just starting to wear out, or should I be looking for some other underlying cause?