Hi Mark,
I've got some interesting news for you....
I hooked up the sonar, here are my results:
Measured Distance / Reported Distance
20cm / 20cm
40cm / 40cm
80cm / 77cm
100cm / 105cm
180cm / 170cm
APM 2.6 Board running AC3.2 firmware.
These were very quick and dirty tests, but so far I'm impressed for a $15 item. I tried a wall that was approx 3m away and the figures just bounced around so I'm taking that as it cant really do good measurements that far off. Good enough for me though if I want to do an alt hold and stay above 2m off any object below I'll be happy with that.
I'll do some better testing once I've mounted it on the Tricoper. Was pretty simple to setup just plug and play :-) Not sure why I havent got this thing already mounted on the Tri.
If you are running the AC3.2 firmware the bad news for you is it appears it it isnt the APM firmware, likely to be the piggyback boards firmware.... If your still on 3.1 then I'd suggest upgrading to AC3.2 to see if that fixes it, if not then I'd recommend playing with the RNGFND_SCALING variable, since yours is out by a consistent amount you might find a slightly more optimal value? If you change the APM code to adjust you'll need to keep merging your code in everytime you update the firmware.
Chris