says 18 mph while parked?
There is not an adjustment, one can only figure your speedo has one of several problems. Does your speedo begin to climb from 18mph as soon as the vehicle starts moving? If so this would indicate the pointer on your speedo has moved from where it should be. If not then the speedo needle ( pointer ) is being blocked by something inside the speedo. Either way, or if the problem is something else inside, the speedo is not designed to be taken apart and put back together. This stops one from changing the mileage on the vehicle. I would have another speedo before I tried to take one apart. Of course, if you could find a used car salesman.... HA HA. Wish I had better news.