1998 nissan altima with manual 5 speed transmission is stuck in 5th gear and will not come out. any ideas?

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Did experience this before.... Clutch was ok but the transmission is busted. Got myself a used transmission from the yard and everything is back to it's normal state. You may want to replace the transmission to resolve the issue.
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More than likely the shifter parts or worn out, rebuild it.

You don't provide much information to go on (type of vehicle, type of engine, 2wd or 4wd, etc.) RPM is always going to be based on the speed you're traveling with a standard transmission. My 4 cyl 5 speed Nissan pickup in 5th gear runs at 3000 RPM at about 75mph.

it could very well be a transmission problem it cant get to a certain gear so it jumps to overdrive to make up for it.

i had same problem it is your cam positioning sensor

That is normal,If you do the same speed in 4th gear the rpm's will almost double.

The rpm's will repond faster in a manual than an automatic.

In a manual the engine supplies the power required directly to the gear that you have selected thus the engine rpm's will increase or decrease as needed.

motor monts

Probably ice jammed up in there. Can you get it inside & get some heat to it somehow?

MY 1983 280ZX WITH 5 SPEED MANUAL TRANSMISSION IS SLIPPING AFTER CAR IS ROLLING. EXAMPLE: YOU CAN BE IN 5TH GEAR AT 55 AND GIVE IT GAS AND THE RPM GO UP ON THE MOTOR BUT IT TAKES A SECOND OR TWO BEFORE THE TRANSMISSION GRAPS. FILL LIKE YOUR IN A AUTOMATIC TRANSMISSION CAR. ANY IDEA?