I sent my car into a Porsche garage. The car was obviously driven while it was in their as about 1/4 tank fuel was used. I asked one of the technician and he mentioned that it was possible to remove a fuse on some Porsche model and the speedo will stop working??
The speedo is just a display, the real work is done by the DME. You can pull the fuse and turn off the display, but cannot tuern off the DME. As soon as you replaced the fuse on the instruments, the revised mileage would show.