48 vs. 68 is a substantial difference. Since the OP states they already have AlphaOBD is there a chance they have messed up or accidentally switched some settings (tire size or gear ratio). Depending on exactly where the speed sensors are the axle gearing may not change the speed reading, but tire size would.......however getting that much difference would require you to have the stock 31"ish size tires but have it programmed for like 44"+ tires.
In general it would be very unusual for the factory speedometer and speed sensors to fault out in a way that states you are going almost 30% faster than what the speedo shows. There is a huge liability factor going on there if you had an accident because of excessive speed.....imagine coming up to a sharp curve and thinking you slowed down to 30 mph, but were actually going 50 mph and slid off the road because of it. That is an open and shut case in court against Ram/Chrysler/Fiat/whatever. Not saying there is no way the speedo and speed sensors could do this, just that it would be unusual for them to fault out in this direction and not throw a solid check engine or ABS light, or cause other issues. Typically any unusual reading from a speed sensor at least throws the ABS light and turns off all of the traction control and ABS functions (better to not have ABS functioning versus it causing an accident from a faulty wheel speed sensor....in the early years if a sensor went out and began reading zero it thought the tire was locked up and sliding and would activate ABS, which basically means it applies lesser or no brake force to that tire. Pretty scary hitting the brakes but no brake pressure!). Modern vehicles do a lot of cross-checking systems to make sure signals are reasonable. For wheel speeds it's looking at all of the wheel speed sensors to make sure they look reasonable to each other, it checks the engine rpm and transmission gear versus what speed you are going to make sure they match, etc....
