30 amp dryer plug is hooked to a 40 amp double breaker, is this ok?
The breaker is primarily in place to protect the wiring. So first thing you should evaluate is the size (gauge) of wiring on this circuit. For a summary of normal copper wire usage:


Breaker 50A Wire Size 6 AWG
Breaker 40A Wire Size 8 AWG
Breaker 30A Wire Size 10 AWG
Breaker 20A Wire Size 12 AWG


It is also recommended practice on dedicated circuits like this (circuit breaker via single branch wire run to single outlet) that the outlet be also sized to the wire and breaker.

Now with that said there is no real problem with changing your outlet to a 30A style and changing the breaker to a 30A two pole breaker as long as the wire is 10AWG or larger size. There would be a strong recommendation that you make sure to use a 4-prong type 30A outlet, as opposed to the 3-prong type, so that you get the safety ground connection all the way to the appliance.