BusTalk

[Q65A]

Modern cruisers all can be described as "6x2 tag tandems" wherein the forward rear axle is the driver and the rearmost rear axle is a non-powered tag axle.
Line haul tractors of the 1950's and 1960's frequently had 6x2 "pusher tandems" wherein the forward rear axle was unpowered while the rearmost rear axle was the driver.
Such axle arrangements fell out of favor with the heavy trucking community as powered tandem rear axles ("twin screws") became overwhelmingly popular for reliable traction under a variety of road conditions.
With today's emphasis on saving every drop of fuel as much as possible, 6x2 tandems are getting another, more contemporary look.
I wonder if cruisers ever would employ load-sensing tag axles that would raise automatically when the axle loading dropped below the legal weight limit for which it was calibrated?

http://www.truckinginfo.com/channel/fuel-smarts/article/story/2016/03/the-world-of-6x2s.aspx

[TheDriver]

I believe it has more to do with the drivers being able to drive the trucks or buses with that set up. A friend of mine had a 2 axle tractor he used to pull 45 foot trailers with and he'd use a JO DOG to give it another axle for load carrying and it was naturally unpowered.
He was not the average trucker and he'd make it work. When he got off the main roads and could see a steep driveway coming he's take the air off(air bags) the Jo Dog so he'd get traction when he needed it.

[TheDriver]

I'd say that with the new engines with a lot of torque and less HP the difference in weight would not affect the fuel use as it used to. Also they could monitor the air pressure in the air bags to know if the tag axle was needed. they do that in trucks.