Is there a way to increase air efficiency in the rover?
Yes, several. The first ones, which I'm sure you've encountered, are the ones that make it at all, so you don't have to replace your suits oxygen all the time, and the one that automates it. Then there is an artifact that gives you more resources for everything, including when you replace oxygen in the rover. Last is from the latest update. Not going to spoil exactly how, but it will remove the extra losses of air in the rover.