I'm probably in the minority here, but I've had TERRIBLE luck with those cheap oven thermometers. The cheap ones are just a coil of wire that expands when it's heated up..or so it seems... and they vary a lot.. at least mine did.
I tested mine with my "steady" & "reliable" and "calibrated" Polder probe thermometer (you know, the one Alton Brown uses on his shows...) By calibrated I mean I check it in ice water and also boiling water (I'm only 500 feet above sea level so 212 is about 212).. then I stick the probe through a baking size potato, with the tip sticking out about 2 inches or so.. carefully close the oven door so you don't crimp the cable. (Don't heat your oven over about 350 to be safe) ... and see what temp your oven REALLY is..
Two of the el cheapo oven thermometers (the $3.95 or so things on cardboard with plastic over them you get in the supermarket) were 15-30 degrees off... If you don't have one of those probe thermometers I'd get one.. then first use it to test your hanging or standing oven thermometer for accuracy. Hopefully you'll have better results with the oven thermometers than I did