More than likely tire issues. As simple as tire balance if not possibly a tire going south on you. Tell me more. Does it do it just at 40 or at higher or lower speeds as well, and where do you feel it, steering wheel, seat....and can you do anything to make it more pronounced?
might have misaligned or loose wheels
the cuase of this is ussaully worped rotars
Could be a problem with your torque converter
Cylinder miss, broken motor mount, or vacuum leak.
check your wheel bearings and if they're ok check your cv joints by jacking up the front of your car and rock your tires forward and back to see if they clunk or shake pretty good
Cylinder misfire. Possibly failed or failing engine mounts.
The engine of the car may need to be cleaned or your oil may need to be changed.
58.6ft. per second
have front suspension checked
Normally this is caused by a tire out of balance or a bent wheel.
could be a problem with your transmision, having trouble shifting
first check your balance on you tires local tire shop will usually balance new tires for free or if you have a reciept from where you purchased the tires some tire shops will balance for free if you bought the tires there if tires are not balanced it causes the vechicle to shake at about 30 mph if that's not it i don't knowAnswerIS it a shake from the engine or a vibration that you feel? It very well may be a tire or rim problem, or it may also be an engine miss loose suspension etc.. AnswerIf it snowed recently, and the problem did not exist before the snowstorm, there may be frozen snow/ice stuck to one or more wheels, causing imbalance. Take the care to a self-serve car wash and spray the wheels from underneath the car and in the wheel wells.