Ipod4 vs ipod5 Touch speed accuracy

Have both ipod4 and ipod5. With same settings for car speed checking the ipod5 gives a speed difference of 10 mph. eg Ipod4= 30mph equals ipod5= 20mph. I trust the ipod4 to be more accurate than the ipod5. Ipod4 will also work in low light where ipod5 does not. It is best to calibrate the speedclock to a "vehicle with known speed". Would like to see Ipod5 and ipod4 show same speeds for same settings.

here2test about SpeedClock - Video Radar