Tachometer Signal Voltage Reduction
Use a diode. Yes, it is simple as this.







simulate this circuit &ndash; Schematic created using CircuitLab

The idea is you aren't interested in the height of the signal at all. You are only interested in the zero voltage level versus another voltage level. That's what the diode does. The internal pullup of your µC will pull the input level to Vdd (3.3V, 5V, whatever) if the voltage on the left is higher than Vdd-0.7V (the diode voltage drop).