A couple of questions...

reduceditem

david29dpo and joshm...thanks for your posts. I'm sorry but you are wrong. Take a look at https://www.fueleconomy.gov/feg/factors-shtml for the short answer. This is the official US government site on the subject. Search around the site for plenty good advice on fuel economy. In colder countries where the winter drop in fuel economy is more pronounced this is not a something anyone would dispute. I have lived on and off in Romania for the last 6 years, doing exactly the same 70km journey every day in my jeep in temperatures ranging from -25c to +40c. The difference in fuel economy is very significant.

waterbaby

I've just been out and got the tyres pumped up to what they should be. Thanks for your help everyone. I'll see how I get on in January, and I'll also give the other things mentioned some attention.

I'm going to leave you lot to debate the air temperature thing if you don't mind, as it's way over my head!

philhuff

I'll wade into the cold/warm air debate and say that both parties are right!

In colder conditions, it takes longer for an engine to reach optimum temperatures and efficiency, and that means it's using more fuel. However, once it's there, it should be more efficient, with more 'bang' from each bit of fuel, meaning you can use less fuel for the same level of performance.

Get too far outside of 'normal', though, and it hurts economy whether it's hot or cold.

joshm_2

Okay, I'm willing to concede that the advantages of increased compression ratio may be outweighed by the extra energy required to bring the car up to operating temperaure on many journeys...