Rpm’s at Hwy speeds

[squaredeal91]

Pull diff cover and know for sure

 

[Bextreme04]

100% hitting 4th

100% not... unless you have 4.56's... which you would DEFINITELY not be able to confuse for 3.08's.

Take a picture of your transmission pan and post it here. Go out and drive your truck and record the speed and RPM of every shift. You can calculate the gear ratio from those numbers. I bet you find that you are not getting into 4th

 

[Bextreme04]

Or just zip the diff cover off and check like numerous others have stated. Every single bit of information you have provided points to you having 3.08's and not being in OD though. Whether that is from you not having a 700R4 installed or from there being another issue remains to be seen.

Is this a new behavior? How long have you had the truck? Is there more to the story that makes you so insistent that you have a 700R4 and that you are definitely shifting into OD?

 

[GTX63]

Maybe take a pic of your transmission pan. I have 3.08s and a tH350 with 31s and 60 mph is almost right at 2000 RPM.

 

[TotalyHucked]

Also, what tach setup do you have to verify RPM? That could also not be 100% accurate

 

[Bextreme04]

This is what the shift drop in RPM should look like for a 700R4. Standard converter slip is usually less than 10%, so if your "4th gear shift" around 2400RPM is only ~200-300 RPM, its the converter locking... not a shift. 3->4 shift should be dropping 1000rpm or so depending on where your RPM is when it is initiating the shift.

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[Bextreme04]

Here's an easy test comparison. If you DO have a 700R4, here is an easy light throttle shift schedule for a 3.08 ratio above and a 4.56 ratio below. See which one looks like what your truck is doing.

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As you can see.. the amount of engine RPM lost at the shift is going to tell you the transmission ratios(which should easily identify what trans you have) and the speed at which the shift occurs will tell you final drive ratio. I'm going to assume that tire diameter is correct because that's the easiest possible thing to check and verify out of all of this.

 

[squaredeal91]

Also, what tach setup do you have to verify RPM? That could also not be 100% accurate

Might be on 6 cyl

 

[82sbshortbed]

I have a 454 with a 700r4 and 3.73 rear end on 28" tires and it runs 2,200 or so at 65mph.

 

[Bextreme04]

I have a 454 with a 700r4 and 3.73 rear end on 28" tires and it runs 2,200 or so at 65mph.

Yep, that checks out. 3->4 shift at 3100rpm would drop you right at 65mph and 2170RPM assuming a 6% converter slip. If it stays that way though... you probably aren't getting converter lockup. With the converter locked it should drop you down to ~2030RPM.
Converter unlocked:

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Converter locked:

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[GTX63]

Might be on 6 cyl

Not that I ever installed an aftermarket tachometer without confirming that it was switched over to an 8 cylinder....but I have heard of it happening to others.

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[squaredeal91]

Not that I ever installed an aftermarket tachometer without confirming that it was switched over to an 8 cylinder....but I have heard of it happening to others.

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It could get bumped. Might be column mounted

 

[zinnette]

100% not... unless you have 4.56's... which you would DEFINITELY not be able to confuse for 3.08's.

Take a picture of your transmission pan and post it here. Go out and drive your truck and record the speed and RPM of every shift. You can calculate the gear ratio from those numbers. I bet you find that you are not getting into 4th

 

[Bextreme04]

The bottom of the pan... so that we can see what model it is.

Does that partial VIN stamped in the rail match the truck? If it does, you can also verify by looking at the RPO's listed on the SPID sheet inside the glovebox or on the fenderwell.

 

[CheemsK1500]

Whether you have th350c or 700r4, always double check that your torque converter lockup harness is intact and wired correctly. You can also bypass the automated vacuum actuated lockup and convert to a toggle switch for on-demand lockup. I’ve seen some people use a floor mounted high-beam switch for this. On the pre-1984 trucks this looks kind a goofy though, since you’d have two high beam switches next to each other.

If you have a pre-lockup TH350 or Th400, then disregard. The majority, if not all factory automatics in half-tons from the late ‘70s to 1986 should have locking torque converters from the factory though.