My car has a speed of 70mph on the SatNav when the speedo says 77. That could be by design, or the factory fitted alloys having a different circumference to th

My car has a speed of 70mph on the SatNav when the speedo says 77. That could be by design, or the factory fitted alloys having a different circumference to the factory steel wheels. I peg my cruise control at 76.5 and overtake all day long because other road users are oblivious to their actual speed.