Echo in nov 2010 was 65% new one 55%.I have pvcs and sinus tachy. What would cause the drop in ef? Should I be worried?

Just be sure.... Although ef of 55% is fine we want to make sure that it is not trending down. If you have numerous pvcs such as more than 25000 a day or continued uncommonly high heart rates, it can cause weakness of your heart muscle and may have to be addressed. Please discuss this issue with your doctor. If your pvcs are numerous, you may need a follow up echo in a year to see if ef is stable.
Probably no worries. If you have a significant cardiac history, then there may be a cause for concern, but these are both considered relatively normal ejection fractions and may be slightly different due to the differences in the way the echocardiogram was performed. Sometimes very frequent pvcs can affect ef, but they typically have to be quite pronounced (>25% of the underlying rhythm).
No, that's normal. Not to worry. Ef of 55% is still a normal ef. The ef is variable within certain level depending on many factors.
Would investigate it. Although both values for ejection fraction are within normal range, there is a drop that needs to be explained. In view of your PVC history, i would recommend a 24 hrs holter monitor, since frequent pvcs (usually more than 10%, especially in a bigeminal or trigeminal pattern) can cause the ejection fraction to drop.