Changing rear end?

That set-up sounds about right to me....factoring in for the difference in tire size, I'm running a very similar setup. Mack engine turned up to 487hp, Mack 18 speed transmission, 4.17 Mack rears on 11R24.5 tires. I'm actually contemplating swapping out the 4.17's for 4.42's.

 

My current set up is a 525 Celect + 18 spd n 4.30 rears...but I am mostly offroad...works good for me

 

Those Swift trucks also had a direct drive transmission.. if the OP is looking at running 2.93s with the same transmission used to turn a set of 3.58s, 3.70s, etc., it's going to be a world of difference, and one which I really don't think is going to save fuel.
The advantage of 2.64s, 2.79s, 2.93s with a direct drive vs. 3.42s, 3.55s, 3.58s, 3.70s, 3.73s, etc. is that you don't have the parasitic loss of power turning a 1:1 ratio that you do when you go into overdrive at a .74:1, .78:1, etc. ratio. But this comes at the expense of having less starting torque for taking off, as well. For a general use road tractor, it shouldn't be much of a problem. If, however, you ran the mountains a lot, or you did any kind of vocational duties or a lot of stop-and-go local driving, probably not the best option.

 

I had a classic xl with a 12.7 Detroit 13spd and 2.93 rears and it did just fine. Fuel mpg pulling my spread axle drop deck stayed between 6.8- 7.5 depending on the load and crusing at 62.
Had no problems with pulling power either a friend of mine had a classic 14l Detroit 13spd and 3.55 both of us running lowpro 22.5's and he never could pass me on the hills.
Have to remember they step the trans different for the different rear gear ratios also mine had a final o/d of .86. If I remember right they have 3 different sets of gears for the inside of the 13 to help coinside with your rear end ratio selection for your best pulling.

 

.86 OD with 2.93s? What sort of RPMs were you turning at 62?

 

That would depend upon the tire size. 11R22.5 would have the engine turning somewhere around 1270. Low profile 22.5's would increase the RPMs slightly...11R24.5's would decrease it.

 

I sold the truck in 2010 but if memory serves me I want to say right around 1275- 1300 in 13th.

 

Most 13s have 1.1, a .86, and a .73. Hence a double overdrive. 9 and 10 speeds can be either a 1.1, or a .74 and even some have .73s.

If you are running the 2.64s and higher speed gears, you would most likely want a direct drive, 1.1 transmission, unless you have a 13 or an 18 speed, and want to have a truck that will be geared for about 120 mph.

The reason the switch to running 3.90s, 3.55s, 3.42s etc, with overdrive transmissions, I believe was to take some of the torque off the transmissions. It had the extra leverage.

Been a while. Someone correct me if I am wrong please....