X 2 it is in the ecm. Have you ever free reved the engine out of gear, Or came out of the water at speed.That will set it off.Not sure what type of sensor you are asking about.
If you are talking about the REV LIMITER, it is built into the ECM. When RPM reach a certain number, the ECM starts taking cylinders and fuel away until RPM is back in check.
There is not a specific sensor for this.
I tell ya, my small block gets on the rev limit and I can't tell...well i had a one minute overspeed fault code on the computer, and the manual was saying something about an overspeed sensor. I find it hard to believe a small block can overspeed but what the heck