Gears don't decrease mpgs, what does decrease mpgs is your foot in the throttle as well as the higher rpms you are running while maintaining the same speed. If you're used to cruising at a certain speed, then the rpms will be higher with a higher gear ratio. However, lower gear ratio will bet better mpg in city driving simply due to the fact that you will be in the proper part of the tq band for Mac efficiency more quickly.
I get what you're saying though, the only way to increase or maintain mpg is by keeping current gear or going lower ratio. I would say that 3.73 are good all around gear, that's what I have and generally net 21 mpg combined with a fair share of city and highway