I would go with the 1.7 figure. I have tested this many times. I was working an hour away job (aprox 60 miles) and I would often work late. By the time I got in the car and drove home, the warning light would come home halfway home. At this point in the journey there are no gas stations. I would often make the remaining 30 miles OK. Risky.
Just fill up when you can...!