Please help me figure out what’s wrong

[Chuckanut]

I have a m1028 1986 th400/208/456gears with the stock 235/85/r16 tire size. According to rpm calculators at 3000 RPM I should be going a little over 60 miles an hour. I’ve also been told that there are individuals doing 65 miles an hour at 3000 rpm i’m doing 35 miles an hour at 2000 RPM i’ve been told others are going 35 at 1800. I can feel my transmission shifting into all three gears. My tachometer is calibrated I calibrated it at idle and I revved up the engine to 2000 RPM and had somebody point the RPM calculating gun thing at the crank to see how fast it was spinning and it was at 2000 RPM so it’s calibrated. But all the calculations on calculators don’t match what my truck is actually doing in terms of speed and RPM. I have a gear vendors overdrive on my truck on it as well. You can feel it go into gear. And I’m doing 60 miles an hour at 2800 RPM which I have been told by individuals as well as on calculators I should be doing 75 miles an hour at 2800 RPM. What is my issue? Seriously I’ve been racking my brain for days now and I can’t figure it out. I am I right and they’re wrong or are they right and I’m wrong. My dad seems to think my transmission has been regeared because it was a decommissioned forest service truck. If you guys can confirm anything, or give me any information that would be appreciated

 

[Valley Rock]

75mph with short tires and 4.56 gears ?

I dont think so

Even with an overdrive you'd be at 3 grand plus on the tach

456s are some steep gears for short tires .

I've had alot of rigs with 410s 456s and 513s, the last two not at all highway friendly with short tires and no overdrive, and overdrive only does so much .

We've gotten accustomed to lock up converters lower numerical gears, taller tires and overdrives for some time now, getting in one of these old wood haulers and hammering on down the road takes you back to a time almost forgotten by many now days, the days of steep gears, short tires and direct drive .

 

[Barrman]

3rd on the TH400 is 1:1. I don’t think that can be changed.

What is the gear vendor ratio?
A google says .78. That would drop you to around 2400-2500 rpm at 60 mph with 32” tires. What is the actual measurement of your rear tires?

How are you getting your speed reading?

What gives your tach its signal?

If your tires are actually 31”, your tach signal is alternator driven with a worn pulley that makes it spin/read faster as rpms go up and your cable driven speedometer is off 3-5%. All that could add up to you being right on for your truck and right on for what other people are getting.

 

[nyoffroad]

I had the same truck (without GV over drive ) and put a 700 R4 in so ended up about the same final ratio.
235/85/r16 tires are about 31.5" tall when new I needed new tire and went with what I believe was a 305 or 33" tire and never looked back. I didn't have a tach in that truck so no idea of RPM but according to my son and then my wife I would cruise about 70-75.