Your speedo might not be off at all.
I switched my XJ from 225/75R15 to 30 x 9.5 R15 tires (two tire sizes larger) and then used a GPS speedometer program that I loaded into my Android phone to check the XJ's speedometer.
It was dead on accurate with 30" tires. It must have been a bit off with the stock tires.
I suggest you use a GPS device, or a GPS program loaded into a smart phone, to test your speed and compare to your Jeep's speedometer.
Your speedometer might already be fine with 245/75R16 tires. My guess is that changing one tire size isn't enough to make it off.
Test it and find out.
Warning: I often edit my posts a few times to get them complete or correct errors.