Take a look at the face of you speedo. There is model number just under the center of the gauge. Sometime K10042 sedans) or 9800 (wagons). Go out to Ebay and search Ebay motor's car/truck parts section for Volvo 240 parts. You will find hundreds of parts out there. Refine your search to gauges. There are always entire clusters for sale. Or just the speedo. There is a small gear that fails on 240 odometers. You can buy the gear or just replace the whole speedo. Either way changing out the whole cluster, speedo or gear is pretty easy. The only hard part is connecting and disconnecting the electrical or cable leads in the rear of the cluster. Over time the leads can be stuck on their connections pretty tightly. A little WD40 will help. By the way there are also Ebayer that sell speedos that they will set to the mileage you need to be correct. Lastly, be aware that after 1986 the speedos went electronic and not manual(cable driven speedo). So buy the right year series for your car. 75-81, 81-85, and 86-93.

Buy a Haynes or Bentley manual for your year and model and it will help you with this easy task.

Good Luck