[V8] Poor mileage

[Jeremy Ward]

Speaking of gas mileage...  I was averaging 16.2 mpg (mostly in town)
and this recently dropped to about 15.5 mpg.  

My thinking it that my failed transmission mount (or is it a motor
mount?) is causing my knock sensor to think I am running with 82 octane
gas and retarding the heck out of my timing.

Does this sound like a possibility?  Does anyone know of a way to test
this theory?

I am hoping that it is a transmission mount and the transmission shop
that did my over haul will replace this for me on their dime (as they
either broke it during the R&R or saw that it was near death and didn't
say anything.)  It has been growling ever since I picked it up from them
(a few months back), and is now starting to do it more frequently, for
longer periods of time.