This of course depends on what MPG you are getting. If it is all city stop and go driving it could be as low as 18 mpg, but my 93 turbo wagon has gotten as high as 35 mpg on the highway with a tailwind (converted to M46 transmission and 3.31 axle). Based on this range you could go anywhere from 324 miles @ 18mpg (assuming you can use 18 gallons of the 19.8 capacity - margin for error) to 630 miles @ 35 mpg. Filling up every 300 miles should be safe if the car is in good tune (normal fuel consumption).

The fuel sender which tells the gauge how much gas is left can go bad in these cars but they are usually repairable. On my car the float which slides along two resistors making electrical contact between them became too loose - electrical contact was intermittent and the gauge was erratic (usually read too low). I was able to fix it for free simply bending the contact until it made better contact while still allowing the float to move. Took some trial and error. There are instructions in the 700/900 faq.

Eric