About 2 speed rer ends.....

[BigRed40]

It will definitely make your engine work harder to get them up to speed which doesn't help mileage. Like kyot said though, I don't think there are gearing options available for those 2-speed axles; BUT the Columbia 2-speed rear axles (made for Ford) did come with 4:44 or 4:11 standard rear end ratio with 3:18 or 2:94 for the highway cruising. So if you got the proper (4.11) Columbia 2-speed axle for your 40, and you disengaged that front axle for the highway driving, you could hypothetically pull it off.

 

[Vicm]

You will find them in older big trucks, they are more underdrive than overdrive , I believe. So your high will still be low.
The one in the pic is not 2 speed, I wish It was.
It has 6.17 : 1 ratio, My friend has one with 8:1 also not a 2 speed. But the same trucks had 2 sp in some of them.
So for the older ones I would say , it will be too low for what you want not the mention the witdth and mass.
The experts will correct me, and I don't know much about the newer ones.

 

[marks69cruiser]

Forget about the two spd axle it is near impossible. Go with the tranny upgrade and you will be happy.

 

[qonos]

Thanks for the info, all. It is very much appreciated.