Correct speedo gear

The charts are usually accurate but I have seen some speedos that are out of wack and you have to swap the gears to find what works the best. I use a gps ap on my phone, its really accurate.
My dart sport is a few mph off. I swapped gears back and forth but found its off one way or another so I made it read faster than what im really going to help prevent speeding tickets. Its only about 3 mph but thats as close as it can get.