Apparently this is an inherent issue, sometimes with most brands. In this case, you might
let us know what model you have. Grandvista's have a hub gear wheel while Xcitings have a Magnetic
Pick-up. I had continuous issues with my Xciting until someone mentioned a redesigned sensor on this Forum. I have been able to adjust the new sensor to within 2 mph over the GPS. I had so many issues, I bought a dedicated GPS for this bike. It's so useful if you ever break down in a remote area and end up calling the AAA Service Center clear across the country. I'm afraid you may just have to compensate or estimate your difference and just ride that way. Urbestfriend....