The latter is roughly equivalent to the US's CAFE standards and leads to gaming the system by manufacturers.
The former is much harder to game - either you use the gasoline (and paid the tax at purchase) or you don't. Doesn't matter (to the tax authority) what car you drive - only how much gasoline you burn.
Are you going to create a commission which decides what a "genuine resource usage" is?
Uhm... if it's necessary, sure? I already suggested a starting point: the current state-of-the-art provides a baseline to compare against. If we need a commission to gauge this, then sure, how about have an agency like the EPA or DoE figure it out. We can even give a good name to products that meet high energy-efficiency standards... like, I don't know, maybe "Energy Star"?
"Wasteful" = "using more resources than it genuinely needs". We're saying very, very similar things here. I don't find the difference between them significant enough to be worth arguing about honestly. I'm just suggesting an alternative that I think might be more viable, but if we go with your proposal, that's enough progress to make me happy.