Not sure what accuracy of a GPS speedo is, although uncorrected, I believe they read just a touch low as they are measuring the speed between a group of fixed points and roads are curved. I'd suspect there is some correction built into the GPS unit, so no idea what the result is.
Try having a word with the trading standards people or a local traffic officer. Good chance there is a measured mile very close to you and it would make sense to check with this as well.
