How much bigger tires? I have found that going up 2" taller just makes the speedo more accurate. I found that to be the case on my LJ and my XJ.
Before you spend time, money, or effort figuring out any more about it, put on your larger tires (and do any regearing you plan to do), then use a GPS device (I used my smart phone with a GPS app) with a speedometer feature in it. This will tell you your actual speed in real time. Compare that to your speedometer to determine if it is incorrect, and if so, how much.
After that, look for a solution, if you need one. You might find you have no problem. If you do have a problem, it will be easier to fix when you know exactly how much it's off.
Warning: I'm notorious for making a post, then editing same post several times, adding info each time, or correcting typos.