Here are some handy calculators:
http://wahiduddin.net/calc/calc_speed_rpm.htm
http://www.wallaceracing.com/gear-speed.php
Have never seen though a mileage versus gear ratio calculator as likely too many other variables come into play. For the effort, a switch to 3:50's is only a 6% reduction (3.50/3.70). A 3.0 gear would IMHO be better as that would reduce rpms by approx. 19%. IIRC, many auto equipped '63's and '64's came with 3.0's from the factory...my '63 352 2-door Galaxie did at least.
Also, an open versus posi (locker, Trac Loc) would make no difference in mileage nor rpms in high gear. It would be cheaper too to swap in a 3.0 open chunk too.....given that you would switch back to your current 3.7 gear chunk once you go home....assuming too you wouldn't fall into the clutches of one of those CA beach honeys and never come home! LOL.
