Just to add to Sponners reply, an increase in Tyre size will also affect the speedo reading too.
For a more accurate speed reading use a sat nav, I have tried quite a few of them over the years and have found the Nav Man to be a quite accurate tool.
I have a mate who was doing what she thought was 100 bang on as that is what the speedo was telling her, but she had borrowed my Nav man which was telling her 93.
When she got pulled over the officer said she was doing 93 but my mate told him that she was sure she was doing 75!!!!
El