If your mopar is you daily driver!
My Challenger is still my daily driver, after about 5 years and over 40k miles. It has a pretty stock 318/904 in it, although I added a 4 barrel and headers and I run 3.23 gears out back. With that set up I get around 12mpg in town, and around 15-16mpg highway. I drive it like a musclecar, so I know I could do better in the mpg department if I lifted a little. :D
Originally I had 2.76 gears in it. With that set up, I got about the same around town, but could do more like 17-18 on the highway.
An 833 OD is a pretty cost effective swap, and with the change I saw going from 2.76's to 3.23's I'd have to say that OD gear would be great on the highway, worth at least a couple mpg's if not more. And the transmission themselves are cheap, I've picked up a few for $100 to $150. Unfortunately, the rest of the parts cost the same as any 4 speed conversion, so those can add up. But having just done a 4 speed swap on my Duster, its actually not all that hard.
As far as how long I'll drive my cars vs the price of gas, I look at it like this. I own all my cars free and clear. Insurance isn't bad, and registration is a fraction of what it would be for a new car. Plus, I do all of my own work. A new car might get better mileage, but that new car payment will buy a lot of gas for my Challenger. Add in service fees, sky high registration and insurance, and the price of gas would have to be pretty darn high for me to even consider it. I'd switch over to an OD trans or drop in some new 3rd members first, and then I can always ride my motorcycle more. Heck, I'd even consider going back to a 2 barrel and manifolds if it got really bad. :shock:. But a new car? Not so much.