......it looks that way. The only way to really be accurate is to use calibrated equipment to measure the actual fuel consumed over a measured distance.
I tend to average it over multiple fillups in order to level the results, which tend to be within 2mpg of each other, as driving style and speed changes between drives depending on the type of journey, (urban or motorway).
My dad managed to get 407miles to a tank before needing to fillup once and I simply didn't believe it, so went out and tried it myself and made it to about the same mileage before the reserve light came on, so ended up at just under 415miles to a tank before reaching the garage. I filled it right up, till I couldn't get any more in the tank and could only get 36.8litres in the car, (don't know the price but it's irrelevant). There is a ceiling, as there will always have to be some fuel in the tank, but I expect one could probably push 430-440miles before it started to miss on braking/cornering but I would not like to be stranded.
There is one thing to note though, reserve lights, (when fitted), come on at wildly different times. I don't know how much fuel would be left in the 42L tank before the pump would run dry, but when I drained my SuperS tank to remove it when I re-built the rear suspension last year, we ran it until the reserve light came on and then drained the tank into an old graduated beer still and found that there was still 15litres in the tank......which seems to explain that, by the time I make it to a garage when the light comes on, the most I can ever get in the car is about 28litres, as opposed to around 36 from my GX.
Anyhoo, that aside, getting that kind of fuel economy is very hard unless you drive like an OAP......trust me I did it for a whole week and it's was quite a challenge!!