Why is my fuel economy 7mpg less in winter?

Why is my fuel economy 7mpg less in winter? Since the weather has got colder, my fuel economy has got a lot worse. From an average of 43mpg in summer, I’m now only getting 36mpg. Apparently it shouldn’t drop by more than 3mpg, at most, because of the cold. Is that correct?

The temperature gauge slowly rises to just under half way, stays there and never wavers. The heater does get quite hot, but never that hot. It has been suggested that this big drop and poor mpg could be caused by a faulty thermostat. Would the diagnostic computer not have spotted this? Do you think this could be the problem? It’s going to cost me £170 for the garage to replace, which I don’t want to pay if I don’t have to.

Asked on 25 January 2018 by Iain Buchanan

Answered by Honest John
Depends how far you regularly drive the car and therefore how much of your motoring is on enriched mixture and how much is on normal mixture. If you are regularly driving 20-mile journeys then fuel economy should not be more than 2-3mpg worse in winter. But if most of your runs are short, then it could easily be 7mpg worse.
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