Fuel consumption seems to be a perennial issue here. Fact is petrol Alfas are pretty thirsty cars, if you have one, you have to live with that. I've got a JTS now, and had a 2.0TS 156 some years back. I never got much over 25 mpg out of the TS and the JTS isn't much better. I don't normally fill cars up, but I always put a fixed number of litres in, usually 20, and zero the trip. Over a dozen or so fill-ups you can work out the consumption pretty accurately, if it bothers you. I never even bother with the on-board computer thingey.
FWIW, my general experience on fuel consumption (and I find this applies to bikes too) is that the biggest determining factor is not how fast you drive, but the length of your journeys.