Speedo calibration - Dakota Digital HDX/ HP Tuners

[NC_John]

Went for my first test drive yesterday and my hdx speedo was reading extremely low (didn't read over 20-25 mph, even at a cell phone gps indicated 55-60). I tried to do the adjust procedure in the gauges but was only allowed a max of 125% which was nowhere close to where I need to be.

I am figuring I need to make a change in the pcm through HP Tuners. I've got a t56 with a 4.56 rear (which is likely the cause of the speedo reading so low). My concern is I've read online that making too large a change in the pcm will brick it. Is this still an issue? What can I do to avoid messing it up? Anybody know? I'll have an opportunity to get into the pcm when it goes on the chassis dyno and make any changes but I need to get it right the first time since I personally don't have my own copy of the software.

Thanks!