Thread: Progress on the bus . . .

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We put a higher ratio diff (lower numbers,) 3.42:1 in the bus because at 90kph (55mph) the motor was revving way too high. (Being classed as an hgv 55mph is the fastest I'm supposed to be allowed to drive on NZ roads.)
Now, at 55 mph I'm revving around 12-1500rpm. Quite satisfactory; I'm happy with that.
But...I have inadvertently exaggerated/exacerbated the differences between 2nd and 3rd, and 3rd and 4th.
Revving too high in 2nd, change up, and too low in 3rd, too high in 3rd, change, and too low in 4th.
So...I decided to split the shifts with a Gear Vendors under/overdrive transmission in the driveshaft.

But; which transmission would suit my application?
So I went to their site:
https://www.gearvendors.com/cg2wdt.html
And decided that the NV4500 would appear to be the best bet for me.
But am I right in this assumption?
I'm not familiar with these things; and I'd hate to get it here (and paid for...ouch!) only to find it's just not right for what I want.
There are an awful lot of people on here who know an awful lot more than I do; and who's opinions I respect...so what do you fellas think?
Am I right?
Or am I wrong?

Help!

(Please?)