Cold weather MPG

[Stirring Moose]

I know there have been a few questions about this recently with people noticing reducing fuel economy in the cold weather. I and others have confidently pointed out that cold air is denser and that more air needs more fuel to go with it, hence worse mpg. Just done some quick maths though and I'm not sure this is the whole story. I reckon that if it's around freezing my 'Valver looses about 12% fuel economy compared to that recorded when it's about 10 degrees outside. Fair enough, but that temp difference only changes the air's density, and thus the engine's fuel requirement, by about 3.6% so where's the rest of it going?! Now I'm talking mpg figures from the MFA which as we all know does talk a certain amount of bollo*ks but can't see why it's level of inaccuracy would be affected by temperature. Anyone got any thoughts?

S.M.

[Christo77]

With regards to the MFA remember that any reading is 'not' a momentary reading..it's an average calculation from the start of your journey.

[MotorPsycho]

Also takes longer to reach operating temperature so cold start valve is open longer, especially noticeable on short journeys. I notice it about the same as you, if it's 10 degrees I'll get an average of 40mpg on my 7 miles to work, if it's cold I'll get 32ish, on long journeys though it's roughly the same but still slightly lower when cold.

[javalin]

>so cold start valve is open longer
nope.  mk3's and later use a longer injector open duration instead.  The diesel may have this though?

James

[MotorPsycho]

ahh fair enough, same kinda principle though

[javalin]

yeah - same end result.

[Agreeable Slick]

Remember that not all of it is going to be burnt, some will just be scavenged out when the ports open.

[Sii]

And in the winter you tend to have the heater on (taking warm air away from the engine), the rear demister and the head lights on which all take energy to power. May also be stuck in more traffic as less people cycle to work.