I have a 2001 pontiac grand prix it shakes when i reach 40 to 55mph and only when press on the gas pedal. i have gotten 4 new tires, and alinement on it and change the arm bars and still shakes.

225/60/16 all 4 tires

Go explain to tire dealership and have that checked. You probably have some tread separation going on.

Turn key to on position without starting,pump the gas pedal 5 times within 10 seconds and wait for the change light to flash, turn key off and your don

bad engine mounts
bad ground point
low fuel pressure causing random misfires

Get all your motor and transmission mounts checked to see if any of them are wore out.

chunk of aliminum in cylinder. checdk plugs for dammage

your car is back fireing.try tune up or timeing belt jumped. get it check out soon

you are apply torque to the engine upon pushing on the gas pedal so the law of nature states the engine mount maybe broken

first take the outer tierod off of the spindle tthen take the boot loose from the rack and try to break the inner tie rod loose make sure when replacing the inner tierod that you count number of times you turn it wel unscrewing the outer tie rid you will need to get a alinment perform after you replace that tierod.

check all fuses for turn signal and brake lights.change fuel filter . first. do a little tune up change spark plugs and wires . if changing fuel filter dont help. have fuel pressure check first before buying a new fuel pump .they run after taxes 400.00 dollars. your fuel pressure should be 41 to 47 psi. replace fuel pump relay also.