An inaccurate speedo won't cut any mustard as a defence - you were either speeding or you weren't, regardless of what the car was telling you. However, a kindly Magistrate might listen to this in mitigation and set any fine accordingly...but don't wager on it if it ever happens to you.
These days even non RPU (traffic) wagons are often found with calibrated speedos, obviating the need for specialist speed measuring kit.
Regards,
PC DD.