That's exactly what I was arguing, yes. That $y is important and $x is irrelevant.

> They can't opt out of the road part of the service charge and build their own roads.

That has no real bearing on the economics of the situation. Paying $x to the government for roads and paying $x to a private contractor for roads work the same way. That's why the net subsidy is $y.

If there is a tax that's applied specifically to gas companies that isn't directly paying for infrastructure they use, then that tax can be subtracted from the subsidies. But the gas tax doesn't fit that bill; if anything it undercharges.