what's better?
 

automatic transmission going 2k rpm driving conservatively or.
manual transmission shifting at 3 1/2 everytime?
when trying to get more miles.

More details would help. If it is the same car with similar gearing I would guess the manual would always win if driven properly. Why would you be shifting at 3500 rpm? I usually shift at 1500. 3000 when need to get up to highway speed faster. Again, the more details you give, the better answers you get.

Automatic at 2,000rpm.

Edit: I agree with everything landon said, too.

The manual usually only wins because you can short-shift. You could make your own transmission controller for an automatic that had some 'intelligence' programmed in and probably match or beat manual numbers with WAY less daily effort.

There are plenty of new cars out there that have better in-town or highway EPA numbers with an automatic. It just depends on what logic the transmission uses and how it's geared.

IMO a 5-speed auto and a 5-speed manual with the same gear ratios and your average fuel conscious user (no hypermilers) the auto could get better mileage(probably significant). On the freeway they'd most likely match cc for cc on consumption.

The auto would also need to have a very good torque converter with very aggressive locking. Autos also weigh more but that doesn't make that huge of a difference, IMO.

An automated manual such as a SMT or DSG could probably significantly beat both if programmed well or just operated well.

Don't auto transmissions lose some power as compared to manuals as well? This could also be a factor in FE. It used to be that autos were always worse than manuals, but these days modern cars often have the same or better with auto. Dunno why that is.

Yup, they weigh more and the torque converter is a bit lossy. Improvements in both of those are why the gap is closing.

The reason why autos were historically worse is because they were much heavier, complex and lost power via the torque converter. Modern Auto CVT's should yield better FE than a properly driven manual but I've never seen any tests that prove this (just haven't looked).

If you're buying a used car your CVT options are pretty limited. In terms of FE, a manual (driven properly) will always beat out a traditional auto transmission.