BEN 470
I can share with you some and maybe offer a little insight RE: a straight truck equipped with a tag axle.
It is of course possible to install a tag axle on a 33k gvw straight truck. Positioned close to a live axle, in most configurations this'll increase a truck's gvw to 56k. In a perfect world, one would expect to generate more gross with the increased capacity. I'm not sure this is the case.
I operated a tag-axle-equipped straight truck for a little over two years; leased first to Tri-State, then to Con Way NOW. With both carriers a higher freight rate exists for loads in excess of 13-14k.
Some time back, Tri State rolled out what they told us was a new computer-assisted dispatch system. Among other changes, they created a special category for their leased 56k GVW straight trucks, began referring to them as "E" trucks, and began attempting to dispatch them on "E" rate loads only at $1.35/mile. Trouble was that the "E" rate loads were few and far between (days and days between, for months!!!). When many operators complained about too few loads, Tri-State allowed as to where we'd also be eligible for the "D" and "C" rate, $1.15 loads that had previously been our bread and butter.
Sadly, Tri-State's then new computer dispatch program apparently refused to effectively blend the "E" units on their dispatch boards with the "D" and "C" units. When a "D" rate load became available on a certain board where a 56K GVW "E" truck had been parked a while, the load would be offered to the first eligible "D" unit instead, failing to recognise an "E" unit had been on the board longer. In response to many complaints, Tri State attempted to put a band-aid on the problem and claimed the issue had been resolved. However many of us felt we were still being "dispatched around" in favor of "D" units (caught 'em red-handed myself, twice!!). Quite a few of us felt we needed to lease elswhere. In my experience with Tri State, I don't feel I made any more money with a tag-axle-equipped straight truck.
The exposure to an occasional "E" rate load didn't make up for the lost "D&C" rate loads.
Con Way NOW, who I'm currently leased to, also has a higher rate for loads in excess of 14K. However, in a little over five months of team driving with them, I've only been offered one load over 14K.
As such, I can't claim the my tag axle is making me any more money than if I were operating a truck without one.
For me, there IS a compelling reason to install a tag axle on a 33k gvw straight truck: That's in support of a chosen lifestyle on the road. If one desires to drive a single-axle straight truck with advanced creature comforts, for instance a six-cylinder engine, an auto shifting transmission, a large condo type sleeper, nicer seats, exterior storage boxes, APU's and etcetera; and maybe a few toys in the cargo box; AND if one desires to carry more than 9000 lbs of freight, a tag axle becomes a necessity.
A modern, well-equipped straight truck can weigh over 22k empty. Without the addition of a tag axle, one simply can't scale much freight.
Good Luck