punto speedo problem

I have a p-reg punto 55, and recently the speedo stopped working. I took it to my local garage where they fitted a new sensor. Since then the speedo reading has been wrong, giving a reading of

40mph when I'm doing more like 50. It's as if it's calibrated wrongly. The garage told me there is only one type of sensor for a punto and there must be a problem with the speedo head. So they fitted a new speedo head but this has made no difference. They are now muttering something about the gears but I'm not convinced they know what they're talking about. Any suggestions? Susan