Should you buy a used confession from this man?

The short answer: Not all of it.

Floyd Landis’ decision to come clean about his own drug use — and throw dirt on Lance Armstrong and others in the bargain — sounds like something boxing promoter Bob Arum once famously said trying to set the record straight: “I’m telling the truth today. I was lying to you yesterday.”

Landis now admits he rode dirty. Every day for the last four years, he swore he was clean.

Nobody likes a rat, especially one whose motives and timing are this suspicious. Despite writing a book titled “Positively False” to protest his innocence and raising nearly $1 million from well-intentioned suckers to mount a defense, Landis is now desperate, largely discredited, mostly out of work, and running out of things to sell. So if nothing else, expect to see a sequel titled: “Maybe True.”

Landis said he came forward now because he was tired of living a lie. He said the only way to prove he isn’t lying now is to provide all the details about his use of performance-enhancing drugs, no matter who gets hurt in the telling.

Then again, Landis said many of the same things while he was still living the lie. And back then, he stood idly by after his manager tried to blackmail former Tour de France champion Greg LeMond — who had confided his history as an abused child to Landis — into testifying favorably on Landis’ behalf.

All this proves is that he’s good at throwing heat.

And light?

Not so much.

Anyone who listened to Landis’ varied and occasionally hilarious explanations for his positive test should have known better. Yet a few things he’s detailed jibe with what we already believe about cyclists and doping — that anyone from the guy at the back of the peleton to several former champions could very well be juicing.

But he also knows the only one most people outside the sport care about is Armstrong. Without his name in the mix, Landis’ revelations would still be languishing somewhere in the sports briefs instead of the headlines. And barring some proof, that’s likely where they will be relegated soon.

I’ve written this before, but it bears repeating: I have no idea whether Armstrong is clean, despite having interviewed him at length a handful of times and witnessed his first Tour de France win in 1999 and his last three in person.

Common sense suggests anybody who wins seven titles in the dirtiest sport of all — sorry, baseball — must be dirty. Several former riders on Armstrong’s teams have been caught doping and then there was his long association with Michele Ferrari, an Italian doctor who’s managed to stay out of jail despite several drug-related scrapes back home.

Still, Armstrong has never been busted for a positive test in a sport that catches more cheaters — including the stars — than any other. He was already a world-class triathlete at 15, and that was before cancer and arguably the toughest training regimen undertaken by a human transformed him into something resembling a cyborg.

He also boasted the most money, best team, best support staff (including lawyers), biggest sponsors, state-of-the-art equipment and a pain threshold his oncologist still marveled at years after Armstrong left his care.

In short, Armstrong availed himself of every advantage the rule book allowed, often in triplicate. And whether that was enough only he knows.

But know this, too: You do not want to get into a he-said, he-said with the man.

Whether as plaintiff or defendant, Armstrong has won every court case he fought, a point he made again Thursday. He’s also survived questionable attempts to nail him by French anti-doping authorities, several damaging books written about him and all those shady associations — and won just about every case he’s contested in the court of public opinion, too.