Truck is less gutless at 2000ft elevation?

[Strelok]

I use my truck exclusively for transporting my motorcycles to riding destinations.
I've taken it over Grant's Pass on I5 three times now, lots of elevation changes.
It seems that up in the mountains I don't really have to shift out of 5th on anything even close to flat and the truck will do 80-85mph in 5th with my foot on the floor.
Don't get me wrong, it's still gutless, but on relatively flat surfaces I don't have to think about shifting

Once I get back to the bay area closer to sea level, where things are much flatter, the truck will not do over 65mph in 5th with my foot on the floor and I often have to downshift just to maintain highway speeds on the slightest incline.

I'm wondering if this ghetto cold air intake the PO put on is causing me to run lean, or what else it could be.
To be fair, by the time I get back to the bay I've been running for 8 hours, so maybe it's heat soak or something.
My truck is a 1990 STX on 31x10.5 tires, supposedly 60k on a rebuilt 2.9L.

 

[adsm08]

First, what rear gears do you have?

Second, if an elevation change gets you better power you may have a MAP sensor going back and giving screwy readings.

 

[Strelok]

First, what rear gears do you have?

Second, if an elevation change gets you better power you may have a MAP sensor going back and giving screwy readings.

I believe this means 3.73 gears.

 

[adsm08]

Looks like 3.73 to me.

3.73 with 31s may be a bit steep for a 2.9. Mine was much happier with 29s and 3.73s.

 

[Curious Hound]

Yes. 3.73 / limited slip

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[Strelok]

Looks like 3.73 to me.

3.73 with 31s may be a bit steep for a 2.9. Mine was much happier with 29s and 3.73s.

I'm sure it would be but they came on the truck and it looks mean, so I'd rather not downgrade.
The tires are 80% anyways.

 

[Curious Hound]

That truck looks great. It would run pretty happy with 4.10 gears if you want to keep that tire size.

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[2trux]

From the looks of it that is a hot air intake. I would look for a factory air filter box to put back in.

 

[adsm08]

Right, I'd be thinking about 4.10 gears.

And ditching that hot air intake.