Superchips, Predator and Hypertech all make hand held programmers that can adjust your speedo. I'm not certain if they all can read diags though. GM's Tech II is not readable by some pure code readers (non-programmers) because it's somewhat proprietary. Well, at least not the less then $100 readers anyway -- I can't say for sure what some of the more expensive ones can do with it.
I dont know about the others but I have the predator and the options are relly good you have 45 65 85 and 120 hp settings you can change your speedometer for tire sizes read diagnostics clear codes and get real time data on the engine I get approx 20 mpg on highway at 75 and am very happy with the programer
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