Note that the NSG370 is a bit different than the two 5-speed transmissions. It has a noticeably lower overdrive ratio (0.84) than the AX-15 or NV3550 (0.78 or 0.79). As such, the gearing recommendations are a little different as well, and generally about a single step different.
I would not recommend going past 4.88 for a couple reasons:
(1), with 4.88 and 34" nominal tires, you're already running about 3,000 RPM in 6th gear. Going much higher isn't going to net any significance.
(2), beyond 4.88, 4th gear becomes less useful at freeway speeds. With 4.88 and 34s, 4th gear is running about 4,500 RPM at 70 MPH. 4,500 happens to be very close to the peak horsepower RPM, and beyond that RPM, you actually lose power. You can upshift to 5th, but you still won't have as much power as you did in 4th.
This logic does not apply with the 5-speed, and in that case I would actually recommend 5.13. The 5-speeds both have a noticeably taller OD ratio that makes up the difference, and also may only have two gears that are effectively used on the freeway regardless, unlike the NSG370, which has three.
With a 5-speed, think of your gear options on the highway as:
5th gear - cruising gear
4th gear- hill and passing gear
But with a 6-speed, you have an additional option:
6th gear - cruising gear
5th gear - mild hill gear, slow passing gear
4th gear - steep hill and fast passing gear
With 5.13, you'd really start losing the usefulness of 4th in the 6-speed scenario, meaning passing and hill climbing actually becomes noticeably harder at freeway speeds. However, with the 5-speed, 3rd wasn't really an option in the first place, so you might as well plan on using 4th for hills and passing.
That said, I do recommend 4.88 over 4.56 in this scenario. That would be similar in OD performance to a 5-speed and 5.13 gears.
