The calculators only give a rough estimate. Mathmatically the speedo should be 6-7% off assuming 235/75.
A more accurate method would be to use a GPS or use a stopwatch and time how long it takes to cover one mile using the highway mile markers (gotta be level to maintain constant speed though). Or trail someone that has an accurate speedo. Using these methods I figure mine is actually 10% off.
Well I just got back from a small road trip with some friends. My speedo read exactly 80 and when we stopped for gas I asked my buddy who i was following what he was cruising at and he said 87 so what is that about a 8 or 9 percent increase, remeber dont look at it as a certain mph off it is a percentage of your speed.