A number of possibilities which will need a more thorough analysis. Poorly balanced wheels and bad wheel bearings would be my first two suspects with the information I have.
It would be highly unlikely.
possibilityThe car is out of alignment or you have a wheel out of balance.
the spare tyre.
Divide the distance travelled by the speed...
A good rule of thumb is to keep a distance of one car length, about 10 feet, for every 10 mph of speed, so at 40 mph, you should be 40 feet behind the car in front of you. Obviously, this is harder to do in heavy traffic.
If your car has a manual speedo, it can be in it or the cable. Noises have to be heard to be correctly diagnoised.
To determine how long it would take for someone driving at 80 mph to overtake someone driving at 50 mph with a 1-hour head start, first calculate the distance the slower car travels in that hour: 50 miles. After the head start, the faster car is closing the gap at a rate of 30 mph (80 mph - 50 mph). To cover the 50-mile gap at this rate, it would take the faster car approximately 1 hour and 40 minutes (or 1.67 hours) to overtake the slower car.
At 40 mph, a car is traveling at about 58.6 feet per second.
The body wants to continue going 40 mph.
It would depend on the car you're driving and if you are driving up a grade or down a grade or on the level. On a level grade probably 40-45 mph would work.
Tires and/or wheels out of balance?
Check wheel balance first.