I've needed a new thermostat in my 2003 Passat for awhile. The engine has been running 30-50 degs cooler than normal (at least according to the gauge). The replacement is getting done now.

But what I've noticed is that for months now my MPG has been below normal. I know that many things affect MPG and a colder engine is one of them, but my question is how much of a difference would 30-50 degs on the cool side make? I've seen my mileage go down about 2 MPG since this thermostat problem came about.

Also, a related question, do cars get better MPG in cold weather or warm (all other things being the same)?