93 cx hatch.
Before I drop the tank and i want to see if anybody have this problem before.

I get like 150 miles per tank before the car start to die out of gas (fill up is only 5 gal).
unless the previous owner put a 5-6 gallon tank?

I check under the tank ----> NO dent

 

I think that you should first pop off the maintenance lid and inspect the fuel pump.

 

before the pump was bad so i swap an good pump from 00 civic into it.
it seem to not pickup gas after 5ish gallon has been used up.
I'm pretty sure it's a 8-10 gallon tank.
so it seem to that pump is not picking up when the gas tank goes down to 4 gallon?

I guess I'll wait until it goes down to 4 gallon and inspect the pump/tank why its not picking gas at that level.
And I just filled up...

 

I think you should double check your installation.