The US gallon is smaller than the imperial gallon, so I would concur that your calculation is accurate in the US.

In case you are wondering, the remaining kms left guage goes down in increments of 10, and when it goes lower than 10, it reads ----. Then you can start looking for a gas station.... If you want to push it further, you can usually hear the fuel pump whirring at a higher pitch and the car starts to surge going around corners - then you have about 5-7 blocks left, but that is to the very extreme end of empty.

If you are on the highway or a large freeway - then use your judgement.