AVR-Chat

On the mega48 part it is much the same. the internal RC is quite good 
over voltage and temp.
BUT the osccal from atmel is only within something like 10%
  the data sheet will tell you.
some parts the number is pretty good, like 2%
I have not looked at the 162 to see what the spec is but I bet it 
is 'large'. If you try another part - it may be right on.

there is a pretty good appnote on how to use the STK500 to set the 
osccal for the voltage selected (and for whatever temperature you 
calibrate it at)
that has worked well for me - you have to use a batch file, but once 
you have it - all is good.
let me know if you want it - I can send you mine.

HTH