I k now this is going to sound weird, but if you actually do the sums, and fuel where you are is similar to here 83.9p per litre, I calculate that you are getting over 38 mpg, which ain't that bad if you are town driving or thrashing it.
Now Fiat guages are just that, gauges. The only way to check it properly is to fill the tank to the neck, run the car and refill back to the neck again, then calculate the consumption properly.
Cheers
D