Although I only have about 2500 miles on my car ('05 HCH MT), I've noticed a large indicated MPG dropoff (high 50's down to high 40's) when the weather turns cold (in the 20's).

This is on a single tank of fuel - so I can't blame it on geting a batch of oxygenated gas?

What's up ?!? - is the ECU enriching the fuel ratio to prevent detonation and if so has anyone run high octane fuel to see if the ECU can compensate?

Any other thoughts? One other data point, I'm not running ANY HVAC - I simply leave every off in either case.