Furious school-bus companies sued this week to block new city rules for hiring drivers. But taxpayers have their own reason to fume: They’d get stuck with the bill.

The new “employee protection provisions” would force companies to hire from a seniority list, pay workers their top previous salaries and contribute to an underfunded union pension — costs they’d wind up passing along to taxpayers.

Mayor Mike Bloomberg scrapped those rules after the state’s top court found them illegal in 2011 — saving $400 million a year.

Yet Mayor de Blasio promised Local 1181 of the Amalgamated Transit Union he’d restore them. As a start, he handed tens of millions to one company, Reliant, to pay drivers more.

Then again, the firm’s Texas owner then ponied up $100,000 for de Blasio’s candidates for state Senate. And, hmm: What was supposed to be a one-time handout became a yearly one — over City Council objections. The total gift’s now over $136 million.

Now the mayor’s looking to restore the cost-boosting rules in upcoming contracts.

It’s nuts. For starters, they’re still illegal, since the top court’s ruling remains in effect.

And they flush vital taxpayer money down the drain by increasing contract costs with no benefit to taxpayers. (No, drivers on a seniority list aren’t necessarily better or safer than lower-paid ones.)

Think about it: The city is resisting Gov. Cuomo’s demand that it match the state’s one-time, $400 million funding for subway repairs. Yet de Blasio aims to give away $400 million every year to please his union pals.

Sheesh. Hizzoner escaped prosecution for corruption, but that only seems to have encouraged him to take more ethical leaps.