Rpm’s at Hwy speeds

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zinnette

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Everything looked at besides pulling the diff cover to confirm says I have 3.08 gears. Running a 700R4 and 26.5” tire and all the calculators show I should be running much lower rpm at 60mph. Currently running 2400 rpm so I’m at a loss as to what gearing I really have or if I have an issue. Figured I’d let the experts weigh in.
 
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I assuming the trans has all 4 gears and lock up in 4th.....if so sounds like you have 456 gears in the back
 
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Assuming you have all four gears of the trans functional, you could jack it up and count the number of driveshaft rotations to every rotation of the wheels. Or, pull the cover and use the opportunity to change the fluid and do an assessment since it sounds like you haven’t been in there- and take a look at the numbers stamped in the ring gear. Do the division and you’ll have your answer.
 
Bextreme04

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zinnette said:
Everything looked at besides pulling the diff cover to confirm says I have 3.08 gears. Running a 700R4 and 26.5” tire and all the calculators show I should be running much lower rpm at 60mph. Currently running 2400 rpm so I’m at a loss as to what gearing I really have or if I have an issue. Figured I’d let the experts weigh in.
Click to expand...
Have you counted the shifts to make sure you are getting all the gears? The 700R4 has a pretty good spread, so you should be able to feel/hear each shift pretty distinctly. That RPM is suspiciously exactly what you would get for your rear gear and tire size without an overdrive. Shifting into 4th(OD) would then get you down to ~1680rpm, then a few hundred RPM more drop when the converter locks.

Just for clarity, that should be three distinct drops in RPM as it shifts from 1st to 4th and then a softer/less pronounced drop in RPM as the converter locks.
 
zinnette

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Did the driveshaft rotations and calculations and it come to 3.08. Do have plans to pull the cover when I swap in a posi this winter. No tq lockup but in OD when cruising. She does not feel like anything more than a 3.55 in her. Had lots of rigs with 3.73, 3.90 and 4.11s.
 
Bextreme04

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zinnette said:
Did the driveshaft rotations and calculations and it come to 3.08. Do have plans to pull the cover when I swap in a posi this winter. No tq lockup but in OD when cruising. She does not feel like anything more than a 3.55 in her. Had lots of rigs with 3.73, 3.90 and 4.11s.
Click to expand...
If you are at 2400 RPM, you are not in OD. Are you sure its a 700R4 and not a TH350? 84 would have been a very early one, would not be uncommon for someone to junk it and go to a "more reliable" TH350.
 
squaredeal91

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Or the 350 with overdrive? Didn't they have those in 84?
 
TotalyHucked

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Bextreme04 said:
If you are at 2400 RPM, you are not in OD. Are you sure its a 700R4 and not a TH350? 84 would have been a very early one, would not be uncommon for someone to junk it and go to a "more reliable" TH350.
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Agreed. Something is off here. I'm gonna guess either OD has failed or it's not a 700. Could be a T350C with "lockup", even though all it usually does is make the truck shudder lol

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Bextreme04

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squaredeal91 said:
Or the 350 with overdrive? Didn't they have those in 84?
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Yeah, its called a 700R4 :)

I think you are thinking of the 350C, which is a TH350 with a lockup converter.


I bet the converter is locking in 3rd gear and the OP is confusing that for the 4th gear shift.
 
squaredeal91

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Bextreme04 said:
Yeah, its called a 700R4 :)

I think you are thinking of the 350C, which is a TH350 with a lockup converter.


I bet the converter is locking in 3rd gear and the OP is confusing that for the 4th gear shift.
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Yes I was thinking of the 350C
 
TotalyHucked

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Yep, if you're truly running ~60mph, you're not in OD

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zinnette

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Bextreme04 said:
Have you counted the shifts to make sure you are getting all the gears? The 700R4 has a pretty good spread, so you should be able to feel/hear each shift pretty distinctly. That RPM is suspiciously exactly what you would get for your rear gear and tire size without an overdrive. Shifting into 4th(OD) would then get you down to ~1680rpm, then a few hundred RPM more drop when the converter locks.

Just for clarity, that should be three distinct drops in RPM as it shifts from 1st to 4th and then a softer/less pronounced drop in RPM as the converter locks.
Click to expand...
100% hitting 4th
 
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