[MetroMPG]

Actually, I think the forum might have been Teamswift.net.

And the suggestions were...

Xtra Frugal injection


Xenophobe Financial independence


X-Files immobilizer


Xtraordinarily Frugal intentions


Xtra Fuel inside


Xample For insight (Honda)


Xtra Friggin inferior


Xtinguishes Female interest

But knowing it was a GM marketer who named the car, and knowing that the XFE designation actually means something on their Cobalt, etc., I'd be surprised if XFI doesn't have a real meaning too...