#1 confirm the actual error with a GPS or interstate mile markers (go 5 miles/5 markers) for the odo & should go (1) mile in 1 minute with the speedo dead on 60 MPH for the speedo & usually when the speedo reads correct the odo will also but if not the speedo head would need to be sent to a speedometer specialty shop tho some people have successfully tweaked em at home. get the right speedo gear in there to correct the odo then see if the speedometer reads MPH correct. there's a bunch for sale on eBay & FABO, not cheap tho but it is important to get it dead on. EDIT meant to say that usually when the odo reads correct the speedo does also