Exhaust smells rich and decreased mpg

[Kermit]

The engine doesn't know what octane you're running. It's only going to command whatever timing is programmed in the ECU. The only way the 91 will give increased MPG is if you've been running bad 87 and the computer has been retarding timing to compensate. Then switching to the 91 would allow full programmed timing.

Thank you. Much better explanation

 

[Bedrck47]

No but it looks like it's on my list. Have the procedure/video somewhere?

Check on youtube cleaning throttle body ford 4.6