Originally Posted by Joe@Dezod

For the ECU speed cut... You need to find the speed sensor going into the ECU. This has proven to be hard because it all goes into a secondary ECU for the EBFD. Only mixed signals hit the ECU from what I remember. BUT, if you can figure out which signal hits the ECU, you'd only have to put a voltage clamp on it at the desired speed. It's hard to know the voltage though at 120 persay, so who knows what to clamp it at. The Apexi RSM though is a smart unti. The problem with voltage clamping is that your speedo will also show only what the ECU sees, so even if you clamp at 120, your speedo never passes 120 as you accelerate. If you use the Apexi RSM (built in variable voltage clamp), your speedo stops working at the desired clamp voltage, but the built-in speedo on the RSM keeps on going showing you what you're actually doing. It also holds the peak value, so focus on your highspeed run, and then look back at it later.

I haven't have time to find the speed wire on the ECU, only the one on the center console. I have the RSM and use it more for suspension tuning than anything (G-SENSOR). It is a capable unit though, not bad for the price.