If you wanna get on it, I'd stick with a manual. If you're looking for MPGs, don't do anything, Because dropping a few grand for a handful of mpgs will take a very long time to recoup. And like stsalvage said, with how things are nowadays, selling is a strong possibility, and you won't recoup the money if you sell it in a year or two. Financially, it'd be better to put that $3k into a gas fund.
Also, manual, auto, overdrive or gearing, the best gas saver you'll ever acquire is a disciplined right foot. I'd stick with the SM420, raise the gears, but if you said low 3s, I wouldn't go higher than a 3.43, if you go any higher like 2.xx you'd have no pep in your step, only benefit if you did like 80% of your driving highway.
I know, I'm practical, it's a drag
