50 MPH = 80.45 KM/H.
Most common (but are not restricted to) causes of noise and vibration problems:
1. A poorly seated tire on the rim 2. The tire or wheel assembly are out of balance 3. Rim that are out of roundness 4. Tire that are out of roundness 5. Irregular tire wear
If there are problem(s) with your tires or the way they have been installed, they will most likely begin to shake and vibrate your vehicle at between 50 and 65 MPH ( 80.45 and 104.585 KM/H).
There's a high chance that the problem may be cause by unbalance wheels, a visit to your local Tire mechanics, to have the tires balance may be able to eliminate the problem.
Check your tie rod ends, balljoints, and wheel bearings. However, it is most likely that you have a bad tire (uneven spots / "runout"), a tire out of balance, or a bent rim. If you have your tire balanced and all of your suspension components are fine and it still shakes, its most likely a bent rim. The only way to fix this is to get a new rim, but that's expensive. What I did was rotate my tires, and the shake went away (bent rim moved to back). This is not the proper way to fix it, but it worked for me.
More than likely you have a tire out of balance. It is possible you have a bent wheel. Have your tires balanced, inspected, and rotated.
Be sure to check suspension components. Namely the tie-rods...
Need more info. Does just the engine run rough or does the steering shake or does the whole vehicle shake betwen 50 and 55 mph?
Normally this is caused by a tire out of balance or a bent wheel.
If it shakes just driving at around 50 mph then the tires need balancing. If it shakes when stopping you have a warped rotor. Edit: tire balance problems are most noticeable starting at around 35 mph. Balance problems can be undetectable by 50 mph. Can also be a bad tire which will be more noticeable usually at slower speeds. Steering wheel shake normally indicates a problem with the front end while a shake felt in your feet and/or the seat will be in the rear. Shake while braking will usually be a rotor problem, however can be a bent hub or spindle and even loose steering parts. Need to take to a qualified tech for further diagnosis.
It might be your transmition...
Get the tyres checked for balance, and bulges in the side wall.
may be due to out of balanced wheels, simply you do wheel balancing. Or otherwise worn out tyre
bent drive axle or axles
An out of balance tire or will almost always manifest itself as a vibration in the steering wheel above 50 mph. A bent wheel will be felt as soon as you get to around 30 mph.
sounds like you need a wheel balance
Steering wheel vibration when applying brakes is usually caused by warped disc brake rotors