Laser optical rev counters (How accurate)

[nickjaxe]

Hi guys I wonder if anybody can advise on this one, my microlight is powered by a rotax 503,

Its just been re-built new pistons ect and the max revs need to be checked and maybe the prop re-pitched as its now reving to 7000rpm, before the re-build it was giving 6700rpm.

The rev counters on these machines are not the most accurate I have been told and I need to check and set it to 6300rpm on the ground,

Anyway I ordered one of those optical laser tachometers that you find on e/bay coming from the far east for amazingly little money,

It came today, but I would like to test it for accuracy, the handbook quotes 0.05%,

For a quick test I tried it on a mains powered electric motor with no load on it, the plate on the motor reads rpm 1420rpm, reading it with my new gadget I am getting 1497rpm, also tried it on the wifes car on which it slightly over reads, checked it on my daughters car also and it slightly under reads,

But the prob is which is the correct fig, anybody got any ideas for checking its accuracy and general comments regarding these meters.

Nick.