Your rims are bent and/or your alignment is off. Bent rim, bad tire, loose suspension/steering component.
Warped rotors
it sounds alot like a wheel balance is needed....
One or more tire out of balance, a bent wheel, or defect in a tire.
Have the tires balanced and rotated.
Could be a small thing like wheels out of balance 40+ "Noisy Inside Car" bad wheel bearings 30+ mph also decelerating harder to control "Steering wheel wobble "on accelerating / decelerating" "Noisy Inside Car" steering linkage: causes the steering wheel to wobble at 40+ mph decelerating harder to control "Steering wheel wobble "on accelerating / decelerating" My brothers car had the above problems I changed his rear spindle, outer, tie rods, "both were shot" bad bearing "blown bearing"
Depends on if it's front wheel drive or all wheel or rear wheel drive. If the vehicle has a driveshaft the U-joints could be failing and that would cause qute a vibration. It could also be a bent wheel or a tire that is out of balance.
If your car makes a wobbling sound at more than 40 mph, the first suspect is a wheel which is unbalanced. Confirmatory signs are that your steering wheel jiggles in your hands, and if you increase speed to about 60 mph wobbling and jiggling stop. Almost any tire retailer can fix this in 15 minutes.Warning. Before you actually try at 60 mph, check that all your lug-nuts are tight; it is probably not this, but if it is 60 mph might kill you.
this is a funny way to answer do yo feel the vibration in your but or the steering wheel. If its in your but there is a problem in the rear wheel assembly, ie bad belts in a tire, wheel bering, flat spot on a tire tire out of ballance
Tires and/or wheels out of balance?
Most common reason is improperly balanced wheels, next would be alignment but that usually causes a pull to the left or right.
I would check to make sure your tires are not terribly out of balance or that one of them a bad spot (bulge) or tread separating. Any of these conditions on the front tires could cause the shaking between 40 and 60 mph.
a tire is out of balance.