I'd say don't bother.
The best saving I ever had was 2 MPG, and that was after a lot of mileage logging. Most of the time there was very little difference.
Average of what 20mpg? so a 2mph is 10%.....not bad
They're not without their problems. You have to remember to engage the hubs regularly to maintain the lubrication splash feed to the swivels- it you forget, then they wear quite quickly and need tearing down to fix.
.True
Steering tends to be even vaguer with the hubs disengaged.
This I really dont understand???
Then there's the scenario of needing 4WD and having to get out to engage the hubs.
Yes you do have to get out
Freewheeling hubs sound like a nice idea but it doesn't really work. Just my opinion. I've got them on my truck and never use em now.
Overdrive, road tyres at the right pressure and a good tune up are far better bets!