Caulk 1/4" gap?
 

After removing the old caulk above a tub, I discovered that there is a 3/16" - 1/4" gap between the bottom of the tile and the tub:

http://paragoninspects.com/images/te..._caulk_gap.jpg

Can I caulk this wide a gap with silicon sealant by using a backer rod, or do I need to first somehow reduce the gap?

Silicone caulk and backer rod is exactly what I have done. Back when green board was used a a tile backer, it was recommended to leave about a 1/4" or 1/2" gap from the tub so water would not wick up into the drywall- the tub has a lip that you don't see, so the water can't get behind the wall. That gap with the tile isn't too bad. If the tile was tight to the tub you might have issues if anything moves/settles.

Thanks.

Here is the reason for the hour-glass shape of caulk made by the backer rod: http://books.google.com/books?id=fDs...age&q=&f=false

http://www.coastalcontractor.net/article/117.html

Be safe, Gary