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I just recently bought a 04 civic dx sedan with around 150k miles and was wondering what cause the floorboard to vibrate at highway speeds? i dont feel any vibration in the steering wheel but i do feel it in the gas pedal. so far i have replaced the tires and the brake pads, calipers, and rotors. any ideas as to what would cause this??

 
Rep Power: 366

there are hundreds of potential causes for vibration in a car.
I do hope you did replace those parts for other reasons than vibration - more investigation needed before throwing parts at it

 
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you are correct i did replace those parts for other reasons. the tires where bald and one had a knot in it and the rotors were warped witch is why they where replaced. i dont think it is my wheel bearings because when i first noticed the vibration i checked them and the tie rods. The struts are not bad or leaking oil. Would a bad motor mount cause the vibration??

 
Rep Power: 221

Potentially, it is also a 20 year old economy car. How fast and what kind of rpms are we talking about?

 
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It’s noticeable around 60mph but it’s really noticeable at 75-80mph unsure of rmp because it doesn’t have that on the dash but my guess is around 3k

 
Rep Power: 221

I'm going to circle back to this....

 
Rep Power: 366

at the age the cars are, could even be the floor rotten starting to fall apart. Anything can happen

 
Rep Power: 223

Possibly CV axles.