Fuel economy and range

[Jarcy]

After two years of ownership, I thought I'd find out what fuel economy I am achieving.
I'm concerned that my range is poor, and I'm having to fill up frequently.
My engine is standard and has just been serviced.

So, after a hoon on full-beans, I achieved 18mpg.
Cruise home at swift motorway speeds, I achieved 25mpg.
Extrapolating, I expect I'm getting 8-10mpg on track, but haven't measured accurately.

OK, so not too bad, and as I'd expect.
So why am I filling up after 150 miles?
I fill up when the needle reaches the 1/4 mark on the gauge, as per the manual, but after a recent fill up I noted that this was 38 litres.
Our standard tanks are 70 litres AFAIK, so I conclude that the 1/4 mark still has almost half a tank full left.

Two questions:
- Am I being over cautious and filling too early? Could I run to the zero point on the gauge?
- What mpg are you achieving following power upgrades? I'm considering going for 425-450bhp next year, but keep the car's nice manners and a respectable range.

[Jarcy]

mr.man said:

I think you will find the tank is 60 litres.

I took the data from here, but willing to be corrected:
http://www.carfolio.com/specifications/models/car/...

The pro-alloy replacement tank states that it's just 55 litres, so maybe 60 litres for a standard tank is realistic.