Don't fear the gear. I have had multiple customers with late model automatics install 4.10 gears and report 2-3 mpg increase in the city. I would recommend a minimum 3.73 for just about anything, 4.10 preferred.

RPM and torque changes are simple percentage, and easy to calculate.

4.10 (new gear) / 3.27(stock gear) = 1.254

1.254 * 2000rshared_pm = 2507rpm

1.254 * 2100ft/lb = 2633ft/lb (wheel torque)

Brian
EFI-Unlimited