There is 0 hacking to install the speedcal. It comes with new plugs. All there is to it is pulling the wires out of one plug and putting them in another. Less hacking than installing a CD Player... Most shops don't even know wtf a Speedcal is. The previous had one in my car but took it out for whatever reason and literally ripped it out so I had to do a SMALL amount of wiring (put in my own connectors) and I have to hook it back up every time it goes in to the shop haha. They literally leave it on the rack for me so I can go in there and plug it up.
To give you a brief idea on how it works, to the best of my knowledge, it goes in between your speedo and where it plugs into the transmission and changes the signal or wahtever to read correctly. If you have any trouble hooking it up, let me know. I spent a week trying to figure out how to do it since my wiring was fudged up.