Drop in MPG average

[bigdog]

I have a '93 Seville 4.9l. As of late the mpg average has dropped from 21.1 to around 15.4. I haven't noticed any difference in driving if I'm cruising. When I first take off at normal speed their is a slight hesitation and feels like I am being robbed of power at about 10 mph. Then it picks up but if I floor it, it doesn't feel like I am getting all of my power. I have a rattle from the converter sometimes( sometimes it is quite loud) Is this causing the problem or is plugs and wires or fuel an issue.

 

[Frat-man-du]

Well could be any number of items. Top 2 (IMO)

1) New 10% 'Ethyl' fuel - dropped my Wife's stang from 20 to 14 and my Mark VIII from 22 to 17-18. This is a recent change, even though my station posted they had the 10% earlier this summer that they really just got it in the past month, as I saw no prior drop in mpg.

2) O2 sensor(s) replace if over 75K