truck revs up while down shifting

[bsb]

On a recent trip while running the interstate for about an hour, I downshifted on the exit ramp and I took notice the truck reved up on its own between fourth and fifth gear. I pushed in the clutch and the truck immediately reved to 4000 rpms. As I slowed to 45-55 mph, I pushed in the clutch and it reved to 3200 rpms. Under 35 mph, I would get 1800-2000 rpms. Whatever is going wrong has something to do with wheel speed because the higher the wheel speed, the higher the truck revs when I push in the clutch. If I shut the truck off and cycle the key, it doesn't do it again. This has happened to me each of my last two trips. The first time I was using the cruise control and thought that might have something to do with it, but the last trip I wasn't using the cruise control. It does not trip the check engine light nor does it log anything on the computer. Any help or suggestions would be greatly appreciated.

[GM-Tech]

I can think of no reason why road speed would have anything to do with engine rpms.

 

If you pushed the clutch in at, say, 65 mph and kept it in as the vehicle coasted down in speed almost to a stop, would the engine speed get progressively lower?

[bsb]

On a recent trip while running the interstate for about an hour, I downshifted on the exit ramp and I took notice the truck reved up on its own between fourth and fifth gear.  I pushed in the clutch and the truck immediately reved to 4000 rpms.  As I slowed to 45-55 mph, I pushed in the clutch and it reved to 3200 rpms.  Under 35 mph, I would get 1800-2000 rpms.  Whatever is going wrong has something to do with wheel speed because the higher the wheel speed, the higher the truck revs when I push in the clutch.  If I shut the truck off and cycle the key, it doesn't do it again.  This has happened to me each of my last two trips.  The first time I was using the cruise control and thought that might have something to do with it, but the last trip I wasn't using the cruise control.  It does not trip the check engine light nor does it log anything on the computer.  Any help or suggestions would be greatly appreciated.

 

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[bsb]

On a recent trip while running the interstate for about an hour, I downshifted on the exit ramp and I took notice the truck reved up on its own between fourth and fifth gear.  I pushed in the clutch and the truck immediately reved to 4000 rpms.  As I slowed to 45-55 mph, I pushed in the clutch and it reved to 3200 rpms.  Under 35 mph, I would get 1800-2000 rpms.  Whatever is going wrong has something to do with wheel speed because the higher the wheel speed, the higher the truck revs when I push in the clutch.  If I shut the truck off and cycle the key, it doesn't do it again.  This has happened to me each of my last two trips.  The first time I was using the cruise control and thought that might have something to do with it, but the last trip I wasn't using the cruise control.  It does not trip the check engine light nor does it log anything on the computer.  Any help or suggestions would be greatly appreciated.

 

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[bsb]

I forgot to mention in my previous message that it's a 2000 Silverado 2500 6.0 L 5-speed manual.

 

Yes, if I did coast the truck to a complete stop with the clutch pushed in, the rpms would come down. When I'm at a complete stop, the truck would be idling fine. Even while I'm upshifting and I shift real slow, I can see rpm gains when I push in the clutch between gears.