I drive mine moderate-to-hard, and get 250 miles with standard unleaded and about 280 with Optimax/BP Ultimate/Shell VPower etc.
These fuels help the performance a tiny bit, but that's not really why I use them. They improve the fuel economy and range, therefore though a tank costs more (an extra few pence per litre anyway), it lasts longer - so it probably saves money, or at the very least costs the same, with a mild performance increase.
The stated 310 is FIAT being optimistic and presuming you'll be light on the throttle. But with the torque curve being as high up as it is on a Sei, if you want to keep a reasonable pace (or often just keep up with traffic!) plenty of revs are required. This can be at least in part remedied with intelligent gear usage, but ultimately most of us end up having to have a fairly heavy right foot. Particularly on the motorway, if you want to accelerate from 60 to 80 or anywhere inbetween, and you're not going downhill, you pretty much have to put your foot right down, spurting tons of fuel.
Manufacturers will always over-estimate the fuel economy, and under-estimate the performance.
I find certain circumstances are more fuel-heavy. When my radiator was going, when my exhaust was disintegrating, and when my induction kit feed tube fell off (whoops
) my fuel consumption got worse. Any of these parts, and presumably more, can affect fuel economy if they require adjustment/repair/replacement.
Also, remember that with older cars like the Sei (most of which are at least 5 years old, most probably older) numerous parts have been replaced with other manufacturers bits, and different cars have sustained different levels of wear & tear. It's no surprise that some on FF have reported very different consumption figures. Also, some people calculate incorrectly.
One last point - the fuel tank can take a lot more fuel than the standard petrol station fuel pumps want to give it. The pump tends to click off at around 31/32 litres. Try removing the pump and letting the tank settle for a second, then trying again. Also, a slow stream of fuel may enter with the pump outside of the fuel tank (just pour it in). I've managed 34 litres before (and I presume the tank had at least a litre as I never run it to stopping!). It's in part about patience. Note how the car's fuel gauge has an area over the MAX point.
