Yeah, I figured this would probably be at least as energy intensive as producing hydrogen, the main advantage here would be that you wouldn't have to scale out a new fleet of vehicles.
Producing hydrogen is currently not as energy intensive, because industrial hydrogen is produced from oil as well. Basicly it works through ripping the hydrogen off and releasing the remaining carbon as CO2, so as a fuel for cars it would be just the same as burning oil, just less efficient.
On the other hand, getting hydrogen from from water through electrolysis is also very energy intensive.
As for terraforming, even if Mars had the perfect combination of gases in the atmosphere, wouldn't it still be way too thin because the gravity isn't strong enough to have a thick enough breathable atmosphere?
Yes the atmosphere is pretty thin and airpressure is lower as on earth, but it shoould be breathable(?). AFAIK scientists don't yet agree on why Mars has such a thin atmosphere. A common theory I knwo is that was basicly blown away by solar winds, as Mars has no magnetic field to shield them off, like earth. Other theories include, of course, asteroid impacts, et cetera.
But even if you had a breathable atmosphere, you would still have temperatures like on the south pole.
It sure seems like this and sustainable nuclear fusion are the perfect couple.
It might be an intermediate solution for a smoother transition to newer technology, but certainly nothing longterm. In the long run, we are probably better off using the power from fusion more efficiently for transportation, instead of using the most energy demanding way possible (excluding short distance teleportation).
Also, if it was a closed circle it would keep the global CO2 level constant, instead of reducing it, which would be better, as the status quo already includes a lot of damage to the environment through raised temperatures.
But then again, we would have to be carefull not to get all the CO2 sucked out of the air through plants, causing a global cooling effect^^.