exactly as you said! every other car I've had, the gauge goes up to 90/middle and sticks there solidly.
with alfas the gauge is a more accurate live reading, so it will move about while the stat opens and closes during your drive (some owners say other things... most say this
if the gauge only hits 70, the stat must be stuck open so the cold water is already flowing about preventing the engine reaching temp, and likely overfuelling, but your mpg is excessive even compared to mine when the stat was bad.
my MPG then dropped like a stone this winter when the temp gauge wouldn't even hit 70, stuck open even wider? my nearest specialist still said 'no its fine!' I gave them the car and said test whatever (not clever) but if you leave the car standing and running of course in time it will hit 90... my problem before was you could see the stat drop as you accelerate! cold air hitting the rad! and then it would even climb during a half hour drive...
anyway, check out the thermostat guide linked above, its great and I'm probably repeating what it says.
if it doesn't fix your MPG enough, I'd assume the Lamda or MAF might be dead. MAF would cause rough idling etc though.
there are guides for both I think.
post back what you find, plenty of smart guys here - welcome to the forum you'll get loads of support