Trying to figure out what T-case high gears I have and the math is not mathin'. (1 Viewer)

[Diesel-Runner]

PO Did a part-time and low gears and OD high gears on the t-case from his word. I can confirm that but doing the math I keep coming out with a T-case high gear OD of around 18% which I am unaware of any OD in that range.
Things a can confirm, GPS 70mph = 2950 rpms with 5.29 diff and measured 34.1" tires.

Chalk measured tire circumference of 213.7 inches.
63360 inches per mile equals 296.5 axle rotations per mile
OD on the 343f is 0.753

So going 70mph = 73877.76 inches per minute, which = 345.7 axle revolutions per minute, which = 1828.8 drive shaft RPMs

2950 engine RPMs = 2221.35 transmission output RPMs

Lastly, T-case output 1828.8 RPM divided by its input 2221.35 RPM equals 0.823~ or roughly 18% OD

What am I missing or doing wrong? Appreciate any help.

 

[baldilocks]

But do you like it??? I’ve built a few various rigs and crunched a lot of numbers along the way. I geared based on those calculations but never really knew what they meant in the end. If I didn’t like the results I made changes.

What is your concern, whether the PO is truthful? Mistaken? Or knowing what you need to do to correct operating parameters to match your desires?

 

[Diesel-Runner]

But do you like it??? I’ve built a few various rigs and crunched a lot of numbers along the way. I geared based on those calculations but never really knew what they meant in the end. If I didn’t like the results I made changes.

What is your concern, whether the PO is truthful? Mistaken? Or knowing what you need to do to correct operating parameters to match your desires?

No I have no concerns about PO at all. I like the way it runs down the road just fine.

Really just trying to understand why there is such a discrepancy. Through researching this I have also found quite a few others that haven't had calculation really match as well. Makes me wonder if the final OD ratio out of the transmission may not be what it is for some?

Unless i have a math error, I either have a .82 OD on the t-case or my 343f has a .68 OD gear

In the end I now have a spreadsheet that is more customized for 80s specifically.

 

[rc51kid]

I dont have any real help for you but my understanding is the overdrive tcase gears only come in 10% lower. I have a set and they apear to be 10% lower with stock ring and pinion. Go figure. Sorry i am no help.

 

[rc51kid]

I dont have any real help for you but my understanding is the overdrive tcase gears only come in 10% lower. I have a set and they apear to be 10% lower with stock ring and pinion. Go figure. Sorry i am no help.
Run them with 315/75/16 and spedo is about 10% slow vs gps with the overaized tires. I imagine stock gears it might be about 20% slow.