unfortunately, I saw little to no change change in MPG. Others may say otherwise, but for me I didn't see large mpg improvements as I would have thought.
here's my experiments for collecting mpg data before/after hub install
Before:
* 2 tanks of 87 octane
* 2 tanks of 89 octane
* 2 tanks of 91 octane
and then the same after.
I drive pretty average (i think?). I commute about 70miles in 1 day, of which maybe 20miles is city, and the other 50 is highway - but that often is negated when I encounter traffic even on the highways.
I really didn't see any variation before and after hub install. That is, I didn't see anything significant. If anything, it was 1 or 2mpg but inconsistently.
To me, significant difference would be a consistent 5mpg, either up or down - which I didn't see.
After realizing there wasn't a noticeable mpg benefit for me, I figured the other benefit of having manual hubs is
(1) less wear on your front transaxle and drive shafts
(2) capability to continue driving if there were significant damage to the front drive shafts or CV
(3) looks nice