I think the key is what mods do you have, what is in the car, city or hwy, what type of gas, what is your tires' psi, do you have heavier than stock wheels, and what speed.
I typically run 90% hwy, 10% city going 60-70 mph. I usually fill up at a Shell (not sure if it has ethanol) or Royal Farms (no E10). With a little taller tires, UDP, CAI, and roof rack removed I have been averaging 29 mpg in the winter and 30.5 mpg recently. I think the car needs a wash and a wax and I could maybe bring that up to 31 mpg. I have some braking mods to install, one being cross drilled and slotted rotors. I figure that might help unsprung weight at the wheels. This should give back a little bit of power and help mpg.
I would also mention that my tires are 36 psi instead of stock 32 psi. You usually want to go higher than 32 for better gas mileage and wear but 36 psi might be too high, so experiment.
I got 34 mpg once, but that was because the car was loaded with two people , it was relatively flat, near sea level, and I had to slow down to 55 mph in one section of VA.
Other mods I would like to do that would help mpg would be VTCS removal, and Customer header with high flow cat.
My recent miles before the light came on was 366 miles. Given that there are two more gallons left, I could have gone another 50-60 miles. That would put me at 416-426 if I ran out of gas (not recommended).
You probably meant to say you could go 630km on one tank.
