4000 rpm at 70 miles per hour

Transmission not shifting into overdrive right.

if the wheel alignment is faulty ( castor wobble ) at 70 mph there is no way that you can describe it as a vibration as it virtually rip the steering wheel out of your hands. If it is a torque converter problem then at around 3850-4025 rpm stationary there will be a vibration of the vehicle and engine The sort of vibration that you are experiencing at 70 mph will come from a drive line problem. If rear wheel drive then it could be faulty "U" joints, worn slip joint or failed rear engine mount
If front wheel drive it may be cv joints or bent drive shafts,wheel bearings or brake callipers not moving freely on the mounting bolts.

check firing order of the plug leads as 2 may be crossed Have a fuel pressure test done on the fuel system check coil/s condition as it may be braking down at revs.

You have a slipping transmission, if the fluid is full then you will need to have the ****** removed and rebuilt.

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Hi

It does look like you have a 3 speed C6, if the PRND21 description is correct. If you do have overdrive, the "D" will be printed in a white oval.

Sounds like it is OK. My Mustang has a 351C with a C6 trans, and 3000 rpm is about right at 70mph.

The defining factor on the rpm will be the Differential gears, which in a truck, might very well be taller gears instead of highway gears, as the taller gears provide more power output at the axles than the highway gears do.

Example:
if you had 3.0 to 1 gears (Highway), that means that the driveshaft will turn 3 times for every complete revolution of the rear wheels(Axle)

If you had 4.0 to 1 gears, then the driveshaft would turn 4 times for every revolution if the axle. Lots of torque for pulling stumps, but on the highway. the engine will have to turn more rpm's to maintain 70 mph vs a 3 to 1.

with the 3 to 1, you will turn less rpm's on the highway, but will have less available low speed power (Torque) at the wheels.

Sounds like the truck is doing fine.

scan it.check vechicle speed sensor.a bad speed sensor will cause shift problems.

go to autozone the brake light switch is maybe $10 and easy to install yourself looks like a plug and is usually located under the brake pedal look brake light switch up on google and should find install instructions thats what i did and saved $100+

I have had same problem I took off the gas cap and then put it back on and it seemed to fix the problem.

Hi, Did you notice the motor actually reving higher when the tachometer was reading a higher rpm.. If it was it could be your transmission slipping.. Set your cruise control and listen to your motor while the tachometer starts moving erratically.. Good Luck