I have a 2004 mazda3 I. Car has always treated me well, but all of a sudden outta the blue it started to run funny. AS you drive down the road you cant go above 35 mph it just wont go. You can sit there and rev it up sounds fine but as you begin to go it will seem fine til you get about 30-35mph then it just stops there wont go any higher. You can drive it all day long at that speed and nothing else seems to happen. Any suggestions would be absolutely awesome

 

Does it gear up all to 5 speed, maybe transmition, tcheck the color if red its good, if black service it!

 

Any check engine lights?
How about the fuel filter being plugged - or the exhaust. Did you hit anything that may have bent the exhaust to restrict it? Or bumped the cat converter to break it apart & plug the exhaust.
Pull the air filter out & see if it's dirty, could have an animal residence in there. As a very quick & temporary test you could leave teh air filter & airbox open to see if the intake was restricted, or maybe the intake is plugged on the 'outside' of the air filter.