Very roughly - to make 1bhp takes very approx 0.1 gallons per hour...
So if it were making the full 240bhp this would work out at 24 gallons per hour.
And if doing approx 120mph that's 2 miles per minute.
If the fuel tank holds say 75 litres that's approx 16.5 gallons.
The fuel would run out in (16.5 / 24) x 60 = 41.25 minutes.
In 41.25 minutes at 2 miles per minute you'd 82.5 miles.
So you're not going to be far off!
In the real world the engine won't be making 240bhp because rpms will be lower than the rpms at which it can make 240bhp and this will decrease the 24 gallons per minute, speed won't be 120mph, if it has the speed restriction and usual top speed it will take less bhp to do the lower speed anyway (even if say it had a CVT) but speed also miles per minute, on the whole though in both a restricted and derestricted Elgrand you'd probably get more than 41.25 minutes and 82.5 miles out of a tank of petrol. Heh I'd still get over 160 miles on LPG and then over 82.5 miles on petrol.