I've noticed over the past few weeks that my Mpg has slowly been on the way down. I check my mpg every time i fill up, and i usually get between 21.5 and 22.5 mpg. In my last four fill ups, i've had between 19.5 and 20.5 mpg.
Could this be related to the colder weather? denser air = more fuel used?

 

I'd say thats correct m8.

 

Yeah deffinately, colder denser air needs more fuel to create the correct air/fuel ratio hence as you say poorer economy.

Thats why you see some tight *****(farmers) in old landrovers with the front grill blocked off so the engine runs hotter.

James

 

Your fuel consumption will increase on short journies but the colder the air supply to the cylinders after the engine is fully warmed through, then the more efficiently it will run and then you have the best chance for good mpg.
It takes a lot longer for the engine to fully warm through than most people realise.

 

Cold weather does adversely affect fuel consumption - also takes longer to get the car up to opperating temperature. However, my mpg seems to have been going up since the summer despite similar driving patterns

Not complaining though