Gear Ratio

[RemCharger]

An ***-dyno would be more accurate.

 

[Mark1969Bee72]

Stick or auto?

Factory 4 speed

 

[Mark1969Bee72]

His claimed 4000 at 50 mph calculates to a 6.40 gear. I kinda doubt it has a 6.40 gear......
Jack it up, find out what gear it has, then find out what is wrong with the speedo, the tach, or the trans.

3.23 rear gears, transmission gears are close ratio. Will need to swap that out.

 

[Mark1969Bee72]

FWIW, 25½ tire diameter, 3.56 R&P gears manual transmission = 3500 RPMs at 72 MPH.
AND
I have to brag a bit, because it's so great: 4.10 R&P same tires, Passon 855 5 speed = 2,750 in .60 5th gear.
Your R&P gears are either 4.56 or one of your gauges (or both) are WAY off.

3.23 rear gears, close ratio trans gears

 

[diesel_lv]

Close ratio transmission gearing has absolutely "Nothing" to do with final drive, which unless it's an o/d, is 1:1.
Just use this.
https://www.reillymotorsports.com/speed-tire-rpm-calculator.html