Whats the optimum cruising speed (I know Impreza owners shouldn't ask!) for a UK 98 Turbo?... ie. what RPM does the Turbo start spinning at?...

It's just sometimes when I'm on my way to work, I think, why the rush? I'm going to work! So I sit in the lane at 60mph taking it easy...

I'm just wondering what speed you can get the best mpg at? ie. at 2,500rpm is the turbo not going, so thats the best speed?