I think Beer Good might have a point with the boost guage, I've got one as well and most of the normal driving is done off boost.

That said, it's pretty easy to tell when the turbo starts to spool up

 

I got 23mpg in my standard MY98 UK turbo, I get 19mpg in my RA import. Always fill up and reset trip counter.

What you have to remember is that speedos over read. You will get better economy if you motorway travel as your speed will be higher and therefore more inaccurate and you think you have covered more miles than you actually have.

 

Best i've had is 375 out of a tank as it hit orange - PPP MY99 Wagon

Worst is 6mpg on track

 

Last Wednesday. Brimmed the tank, did 344.1mls and filled up Thursday morning, same garage, same pump and no air locks in the system. 54.38litres = 28.73mpg which is pretty good. Maintained a fair speed. Not a lot overtook me and there was an engine in the back for half the journey.
95 WRX Wagon around 400bhp with TD05-06, 550s Gruppe-S, Link ECU etc.
Can get down below 18mpg on cold starts, town work and mapping ie hard driving.

STi ver 6 Wagon 585bhp = 17-24mpg but 6mpg at Croft.

 

344.1 miles....you sure you're getting full throttle, Harvey? Or is this the new top secret " twin tank " prototype?

 

Nope 320 is correct.. 160 miles there and 160 miles back.
And the MAF was new in January. AFR gauge and Knocklink say all is fine and dandy