Um, yeah. I assume you went from 31" tires (stock) to tires that probably measure around 34". That's almost 10% difference. So, whatever your speedometer says now, add 10% to that to get your true speed. And your odometer is off also.
If your tires were smaller than that stock, then obviously it's a bigger change.
You need the ProCal and/or a new speedo gear depending on your model year..
~Lots of modded Jeeps and a Toyota 4Runner~
My favorite quote from the press release.. sad but true: "Chrysler claims its products, especially the Jeeps, can handle all sorts of terrible conditions, churning through muddy swamps, climbing mountains, and crossing rivers. Turns out they cannot even handle a car wash."