Been writing a 109 campaign over the last few months so my experience of 109 fuel consumption ingame is based on this experience....
At full throttle consumption seems ~ right, maybe a little on the high side(depends which reference you believe
), but we are able to run high boosts for quite a long time.
What i do notice is consumption at cruise levels seems too low when boost is at lower levels, how much of this is due to being able to run at lower boost/rpm levels to maintain cruise flight might be the cause seems to me the question to be asking?
This leads to a situation ingame where in a 'typical' bomber escort to london, the 109's have more fuel than they ought. i'm also finding a relationship between fuel level, discipline AI level and how long they will stay and fight before running for home. As a result i'm finding using down to~75% fuel level for 109's gives reasonable results- at least for me; ymmv)