My first tank of fuel (since I bought the Vitara) was 26mpg (10.9L/100km) before the manual hubs. I put them on right away and next tank was 29.6mpg(9.6L/100km). 14% improvement. On my first tank I was driving empty and doing safety inspections and crap like that, second tank was hauling around family through the city, and my Dad is pretty big. I raced it a few times on both tanks to attempt to be consistent. I drive 120kph on the expressway (75mph). Next is a K&N which may get me past the 30mpg mark. I figure the $100 investment will pay for itself in under 8000km (5000 miles).
Fordem-- while there is a freewheel disconnect, and there is no load on the CV's, but the boots will still wear since they are turning all the time, and there is still weight that is spinning in the cvs and axles, and it takes energy to turn that weight, just like bigger tires use more fuel, even if you re-gear your diffs. The toyota axle shafts may only have been a little bit heavier than the cv shafts on the vitara, but are still connected to the ring and pinion and driveshaft, and more weight to turn equals more energy required to turn it. I gained 4mpg, not exactly an insignificant improvement. If I had only gained 1 or 2, I might chalk that up to driving habit or weather, but a 14% increase is VERY compelling i think. I don't want to argue, I just put the info here for others who may be thinking of doing the same thing themselves. I have a significant increase. I hope someone else who is environmentally/economically conscious reads this gets similar results.