CINCINNATI — The math doesn’t add up, which might be the only undisputed truth in what has been a strange summer for Curtis Granderson.

In the Mets’ 6-3 victory to complete a three-game sweep of the Reds yesterday, Granderson blasted his 25th homer of the season, a towering drive that landed in the distant batters’ eye in straightaway centerfield. It was a solo shot, no surprise in a season in which Granderson has tallied only 45 RBIs.

“I have no clue what I’ve done historically throughout the course of my career,” Granderson said about his freakishly low RBI total.

Of any player all-time with at least 25 homers in a season, Granderson is on pace to finish with the fewest RBIs. He’s locked in a race for that dubious distinction with the Cardinals’ Jedd Gyorko, who has 51 RBIs to show for his 26 homers.

Until this year, the honors belonged to Joc Pederson (2015) and Ron Gant (2000) — , both of whom blasted 26 homers to knock in just 54 runs. Those marks appear destined to be broken, likely by Granderson.

“A lot of it has to do with being in the leadoff spot and the other thing is we have not hit as a team — and everybody’s included — with runners in scoring position,” Mets manager Terry Collins said.

advertisement | advertise on newsday

Indeed, Granderson’s lack of run production has been a lethal combination of factors. Hitting in the leadoff spot most of the year has deprived him of opportunities to hit with runners on base. Of the 99 players entering play yesterday with 500 plate appearances, Granderson came to bat with the third fewest men on base.

But even when presented a chance to do damage, Granderson has let them slip away. He’s hitting .125 (11-for-88) with runners in scoring position, struggles that represent yet another statistical anomaly.

Even in a down year compared to last season, Granderson is hitting .225/.317/.445, production that ranks him slightly above league average when measured by adjusted OPS.

Yet, he has knocked in only 8.1 percent of the runners he’s had on base, the lowest figure in the history of the franchise. It places him in the company of lightweights such as Frank Taveras, Rafael Santana and Rey Ordonez.

Home runs, however, have propped up Granderson’s production, even though all but four have been solo shots.

Mets videos

Said Granderson: “It’s never a bad thing whenever you can score a run with one swing of the bat, no matter what the situation or score of the game happens to be.”

Perhaps his luck is beginning to turn, which would come at a good time for the Mets.

The arrival of Jose Reyes has allowed Collins to move Granderson out of the leadoff spot. In recent days, Granderson has hit cleanup, which had been the Mets’ original vision when signing him to a four-year, $60-million deal before the 2014 season. In his last eight games, Granderson has smashed five homers with 10 RBIs.

“I’ve got him in a spot where he’s there to protect [Cespedes] a little bit and help to drive some runs in,” Collins said. “And that’s what he’s doing right now. I’ll go back and say what I said the other day: We got this guy to hit fourth. I finally hit him fourth, he’s finally driving in runs.”