I have never used a dial type therm that actually gave me the right reading. Maybe the probes are to short and don't get a good reading on all my thin walled smokers. They do add some bling to the smoker so I put them on anyway. But I just count on digitals in a block of wood sitting on the cooking grate.
webbsa, the only therm I put on my cookers are Tel-Tru they are very accurate as far as therms go. I also place them at grate level using 3" to 6" stems.
There are other colors as well. http://www.teltru.com/s-141-anodized-aluminum-dial.aspx
I will say this and agree with totally with MattS on their accuracy as far as your grate temps. The door therms should be used as a mental calculator as to what your grates are actually cooking at, I have seen accurate door therms be as far as 25° off from the grate temps. I always use oven grate therms on the grates to get the differences in the two then just make a mental note of them. I hope this helps.
What KAM said.
I use the door therms mainly for bling also. A good mechanical thermometer will have the ability to be calibrated. Usually there is a hex fitting on the back of the thermometer that is located at the base of the probe. Turning the hex fitting with a wrench and holding the thermometer housing in the other will change the calibration of the needle. For an oven style thermometer you usually can use a digital probe that has been calibrated in boiling water as a reference for setting the mechanical one.