Had two fills this week (Wed and Fri). 50.56 and 53.52. The only main difference between them was that I adjusted my timing again. I have been getting some pinging/detonation so I stepped up my octane this fill to see if that helps. I have been keeping all my highway speeds to a max of 55-60mph.
The 53mpg was a personal best for me in the Xtiva.
If 53mpg wasn't a bad fill or a fluke...I should hopefully reproduce that (or more ) next tank.
*EDIT* I guess I jumped the gun by saying the timing was the only difference. I have noticed my exhaust getting louder everyday....Well, It finally let go. The down pipe off the exhaust manifold (to cat) just fully seperated. REALLY LOUD NOW! I believe this reduction in exhaust back pressure may have been the mpg increase. It also recently has been having trouble holding a steady idle. This makes sense due to the lack of back pressure. I will not be driving till I put on a new pipe (I have read that running an open exhaust manifold may cause valve damage). Time for some free flowing exhaust???