Here's my theory. It's all about turbo use. I regularly do long motorway runs (I do 40k plus a year) and average 250miles to tank on a 98 sportswagon. However left for Edinburgh at 4.00am and returned at 6pm which is a long f***kin day with heavy meetings inbetween. I was so tired I put the cruise on at 60mph on the A1 run (anyone who knows this road understands that its a speed camera every 100 yds) and wafted home. Ran it with the light on for a while and got 320mls from a tank!!!! I have the front cat removed due to a performance from pipe but I've never done this on any of the two turbos I've owned.

Also in my opinion, blasting round the streets on the limit gives no better MPG than a cruise controlled run at 90mph. If the turbos spinning the economy drops like a stone. Thats my ten bobs worth.

 

This is weird... in my 95 WRX sedan, with only an exhaust mod (and to tell the truth I'm not sure how much of a mod it is) I have regularly been getting 9-11 litres per 100km.
Now I am pretty sure my tank is a 60l tank (can't read Japanese but I can see a nice "60l" somewhere in the manual!) so that would mean I can pretty much get 500-600kms on a full tank, and thats about 350miles.

Either I'm doing the maths wrong, or for some strange reason my car is reasonably economic (admittedly, I've owned the thing for 2 weeks and filled up only 4 times, but hey! Figures are figures...) And this is a mixture of cross-country and city driving fill-ups, too.

Can anyone tell me at what RPM the turbo will 'begin' to spin and at what RPM it is spinning enough to actually make a difference to mpg?