Here is a bit more detail on why I almost always recommend 4.88 for 35s and 4.56 for 33s with the NSG370, and not the next higher ratio.

(Ignore the modified engine curves.)

You have two curves on the above chart, Engine Torque, and Engine Horsepower. Unfortunately, a lot of people conflate engine torque with torque at the wheel. While they are indeed directly correlated, that correlation only holds true when the overall effective ratio of engine revolutions to feet traveled holds constant. Any change in transmission gear, transfer case range, differential axle ratio, or tire size causes a proportional change to that ratio.

Horsepower is exactly proportional to torque times revolutions per minute. Thus, even when ratios or tire sizes change, an engine putting out a given amount of horsepower at a given instant will deliver a certain amount of horsepower at the wheels (neglecting negligible adjustments in drivetrain frictional and viscous losses).

To keep things short, the net effect is such that at a given road speed, propulsive force is directly proportional to the horsepower put out by the engine, irregardless of what ratio or tire sizes happen to be in between (provided that those ratios and sizes allow for the engine to be capable of producing that horsepower at that road speed).

So let's take a scenario. 4.56 ratio, 33" nominal (32" effective) tire radius, rolling up a hill at 75 mph. For simplicity, we'll use the Duratrac specification of 635 revolutions per mile.

6th - 3039 RPM

5th - 3618 RPM

4th - 4523 RPM

4.88 ratio, 33" nominal (32" effective) tire radius, rolling up a hill at 75 mph. (635 revolutions per mile.)

6th - 3353 RPM

5th - 3872 RPM

4th - 4840 RPM

In 6th gear and in 5th gear, you will notice that with 4.88 gears, the engine does indeed produce more horsepower at 75mph than it would with 4.56. 6th and 5th actually produce slightly more engine torque with 4.56s than 4.88s, but due to the ratio difference, 4.88 still delivers more propulsive force to the ground.

However, in 4th gear, notice that not only does the engine produce significantly more torque with 4.56 rather than 4.88, it also produces more horsepower. What this effectively means is that in 4th gear at 75 mph, you will get slightly

* more* propulsive force with the 4.56 gears than you will with the 4.88 gears. In addition, it will do so without bring you quite as close to redline, eliminating a bit of risk associated with holding those high RPMs for a long time.

...but what it you want to use 5th gear in place of 4th?

The answer is you need to gear far deeper than 4.88 to get 5th gear as useful as 4th is with a 4.56 ratio. In fact, in order to bring 5th as close to the peak HP RPMs as possible (4500-4600 depending upon the source), you would need to regear to 5.38 or even 5.89.

Another piece of food for thought: An internal combustion engine generally has peak thermodynamic efficiency when run at 100% throttle at or near the peak torque RPM. As you exceed the peak torque RPM, you still gain additional power capacity, but fuel consumption growth outpaces power production growth as RPMs increase. Essentially, you see diminishing returns on adding more fuel all the way from peak torque RPM to peak horsepower RPM, at which point increasing RPMs has a

* negative* correlation with power output, while simultaneously still

*increasing *fuel consumption.

Since in either case, 6th gear is already straddling peak torque, there is little efficiency difference between the two ratios. In 5th, 4.56 holds a slight advantage in efficiency. In 4th, not only is less fuel consumed, more power is delivered for that fuel consumed. Note that below 100% throttle, peak efficiency RPM roughly scales with power demand. The above efficiency discussion assumes WOT.

So in my opinion, this debate is settled by one simple question. Are you willing to rev your engine at 4500+ RPM for short periods for maximum propulsive power? If yes, go 4.56. If not, go 4.88 or 5.13.

Note the above conversation only applies to 4.56/4.88 relative to 33" nominal tires. Substitute that with 4.88/5.13 for 35" nominal tires.

TL;DR: Y'all need to downshift more.