I bought the tuner first. When I did the 4.10's, I just made the adjustments with the xcal for the speedo. When I did my 180deg t-stat, same thing, made the fan adjustments with the xcal, and so on. I would do mail order 1st, then once you do all of your other mods, get a dyno tune. You can do fuel global adjustments, timing in 2000rpm increments, change shift points(auto), bump up the rev limiter, and set up a 2 step rev limiter for the track if you are using and sort of competition rubber. You can also data log to a laptop, or do live data logging on the unit. I wouldn't hesitate to buy the sct first.