Greg Gania, play-by-play broadcaster for the Double-A Erie SeaWolves, says Rondon, who cracked MLB.com's Top 100 Prospects list, definitely hit 101 mph on the ballpark readings when he was there during the summer, though scouts there clocked him at 102 on their own radar guns.
Mike Maroth, the former Tigers pitcher who is now the pitching coach at Class A Lakeland, said he saw him hit 102. He saw Rondon throw changeups up to 90 mph, and he had to smile.
"I had a hard time getting my fastball up to 88," Maroth said wistfully.
Tigers assistant general manager Al Avila says he saw Rondon top that.
"I've seen 103," Avila said, "and I've seen the most consistent 100s outside of [Justin] Verlander."
Rondon himself claims he doesn't know. He says he usually doesn't look back at the radar readings on scoreboards after he throws a pitch.
"Sometimes," he said through an interpreter.
The hardest he has been told about was 104, which a friend claimed to have seen in a Minor League game against the Yankees. Rondon didn't see it. That might be where the legend gets stretched.
Whatever the top number, it's how common triple digits became that helped grow the legend.
"You were pretty much guaranteed to see 100 every time he stepped out on the mound," Maroth said. "A hundred is pretty common for him. The thing is, it doesn't look like it's that hard before he throws so free and easy. He's not a max-effort guy. The ball just jumps out."
Though others have hit 100 on a rare occasion, it's the kind of velocity nobody has seen consistently from a Tiger other than Verlander since Joel Zumaya was healthy. As much awe as Zumaya's fastball attracted, Rondon's fastball has the potential to be a spectacle, no matter what happens in his bid to win the closer's job with no big league experience.
Even his new teammates are curious. All that most Tigers players have seen from Rondon came from a brief appearance in a game last spring when the Tigers brought him along for a day as an extra reliever.
"To be honest with you, I've never caught him," Alex Avila said during the Tigers' winter caravan last month. "I've never even seen him pitch live. All I've seen is video, and all I've got to go by right now is just what everybody has said and what all of you guys have written. I am as interested as you guys to see what Spring Training holds for Bruce Rondon."
Rest assured, his fate won't rest on how hard he throws. If manager Jim Leyland holds to the approach he has taken during past Spring Trainings, the radar board at Joker Marchant Stadium will be off. That philosophy goes back to Zumaya's days in camp, when Zumaya was known to look over his shoulder at the numbers.
Rondon doesn't sound like he'll get wrapped up in that. What encourages some, though, is how he has pitched in spotlight situations. His Futures Game appearance, his first real prospect spotlight, was one sign. A matchup at Class A Lakeland against Vladimir Guerrero was another. In that case, it wasn't simply about velocity.
Guerrero, once known for the quickest bat in baseball, was making a comeback attempt and making his way up the Blue Jays system. Rondon hadn't built up the name yet.
"He didn't throw a pitch under 100 until he threw one of the best sliders I've seen him throw," Maroth said. "He really stepped up, went right after him."
The slider was 87 mph with late break. The fastball that eventually got Guerrero, Maroth said, was 101 mph outside. Guerrero got his bat on it but grounded out.
In those instances, Rondon seemed to channel his adrenaline well in big situations. At neither point, though, was he auditioning for the big leagues, let alone for the closer job. The closest he has come to that was his late-season stint at Triple-A Toledo, where he struck out nine on five hits over eight innings with seven walks.
The composure is what Leyland will be watching more than the velocity readings.
"You can't make a senior out of a freshman," Leyland said. "I don't know, but I do like talent, and he's got a lot of it. We'll give him an opportunity."
Rondon throws with Zumaya-like velocity, but Zumaya never got a chance to close in Detroit. Rondon throws a little harder, but as Al Avila pointed out, the difference between 100 and 102 mph is arguably trivial. What separates Rondon at this point is his use of his other pitches.
Maroth praises Rondon's slider, a late-breaking pitch that has movement without sacrificing much velocity. Al Avila praises Rondon's changeup, which he can throw with deception in part because he throws his fastball so easily.
"He doesn't throw it very much," Avila said.
Both pitches, Maroth said, Rondon throws for strikes. He doesn't have to rely on hitters chasing them to get outs.
No matter what is to be believed on the fastball, it's the kind of stuff that a lot of current big league closers would love to have. At some point, it's going to dazzle big league hitters this spring. How he uses it, and how he handles everything mentally, will be a big factor on whether he gets to show it on Opening Day, when the Tigers visit Minnesota.
It's the city where Zumaya threw his signature fastball, hitting 103 on the radar at the Metrodome against the Twins as a rookie in 2006. It's also where Zumaya threw his last Major League pitch, collapsing on the mound at Target Field after blowing out his elbow in 2010.
It's where the legend on Rondon could turn to reality.