How do you disable a speed limiter on mack trucks?

The governor is programmed into the motor's ECM. If it's your truck, you simply take it to the dealership, have them hook the computer up, and change the settings from there. If it's not your truck to do this with, you have no business doing it. It could violate the company's insurance stipulations, it will certainly get your fired when your company finds out, and it'll result in a big black mark on your MVR which will make it very difficult for you to find future employment (employers are less than appreciative of having their equipment tampered with). If a fast truck is that important to you, get some experience in, and find a company which lets their trucks run faster.