For the 2nd approach - how do you know the liters capacity and the refill rate?
The correct one is the mix of both. In order to calculate how many cars can pass through a certain station, you need to understand how frequently you need to fill in the tank, i.e. the gas tank capacity and the litters spent per day. Alternatively, you can just use how often you (your friends) visit the gas station on average as a benchmark.