ORIGINAL: foxy42
The guy who said his on-board was way off the gun...it's the GUN that's wrong, not the on-board, unless it has a problem.
On-boards are 'generally' more accurate than the guns, by simple virtue of the fact that you wrap them round the bottom fin of the head, it's closer to the actual engine temperature, guns will normally read lower, as they are being pointed at the glow plug.
Fact is, all these cheap temp readers are pretty inaccurate (or should I say total pants, they're a vague indication at best, unless you get very lucky). You have to spend $200 plus to get a good one and it needs recalibrating by the manufacturer regularly (every 18 months or so). Go to a track and ask a bunch of different people to tell you your engine temp. Bet you get a slightly different reading each time.
My El'Cheapo Duratrax is accurate to with in a few degrees. I've tested it by heating a cooling head in the oven and measure it with a contact thermometer and the Duratrax.