Ever been driving along, everything's fine, and then your car starts shaking when you hit 40 mph? It's pretty unnerving, right? You're probably wondering what's going on with your car. 

Don't worry, though. This kind of thing happens more often than you'd think. 

In this post, I’ll go over the five most likely reasons why your car shakes at 40 mph the stops, and how you can figure out what's wrong.

#1 Unbalanced Front Wheels

The most common reason your car might shake at 40 mph is that your wheels aren't balanced. 

When your wheels are out of balance, it means the weight isn't spread out evenly around the wheel. You'll really notice this at higher speeds, usually around 40-50 mph, because the wheels are spinning faster.

As you speed up, the uneven weight makes the wheels wobble, and you'll feel that through the steering wheel and the floor of the car. 

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It might get worse at certain speeds and then get better as you go faster or slower.

To fix this, you'll need to get your wheels balanced by a professional. They'll add small weights to different parts of the wheel to even things out. 

It's not too expensive and can make a big difference in how your car drives.

It's a good idea to get your wheels balanced every 5,000 to 6,000 miles or whenever you notice weird vibrations while driving.

#2 Torque Converter Shudder

If you've got an automatic transmission, the problem could be something called a torque converter shudder. 

The torque converter is an important part of the automatic transmission system. It's what transfers power from the engine to the transmission.

A torque converter shudder usually happens when there's a problem with the transmission fluid or the converter itself. You'll typically feel it as a vibration or shaking when you're going between 30 and 45 mph, especially when you're speeding up a little.

It might feel like the car is hesitating or stuttering, almost like it's having trouble keeping a steady speed. 

Sometimes, the problem goes away when you get over 45 mph, which is why you might notice it at 40 mph and then it stops.

To fix this, you might need to change the transmission fluid or, if it's really bad, replace the torque converter. 

It's best to have a good mechanic look at it, because if you ignore it, you could end up with bigger transmission problems later on.

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#3 Worn Out Tie Rods, Control Arms, Or Ball Joints

The next reason could be worn out suspension components like tie rods, control arms, or ball joints. All these can make your car shake at certain speeds.

Tie rods connect your steering to the wheels, and control arms and ball joints let the wheels move up and down without messing up their alignment. When these get worn out, your wheels can move in weird ways, and that’s when the vibrations start happening. 

You'll probably feel it the most on the highway.

The shaking can get worse when you're turning or driving over bumpy roads. 

Unlike problems with wheel balance, this kind of vibration might not be constant and could change depending on the road and how you're steering.

To figure out if it's worn suspension parts, a mechanic will need to take a good look at your car.

Once those worn-out tie rods or ball joints are replaced, your car should stop shaking and be way smoother to drive.

Easy fix, but it’ll make a big difference!

#4 Worn-Out Shocks Or Struts

Shocks and struts are really important parts of your car's suspension system. 

They’re there to absorb the bumps and keep things steady while you drive. But if they wear out, you’ll start noticing problems, like shaking or weird vibrations, especially at certain speeds.

If that is the case, they can't control the movement of your car's springs as well. 

This means your car might bounce around or sway too much, which you'll feel—especially around 40 mph, when your car’s weight shifts more as you speed up or hit bumps.

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You might also notice the shaking gets worse when you drive over bumps or uneven roads, and your car might feel less stable in general.

Sometimes, you’ll even hear a clunking sound when you hit a bump.

Getting those worn shocks or struts replaced makes a huge difference in how your car drives. It's usually best to replace them in pairs—both front or both rear—so everything works evenly.

A good mechanic can check your shocks and struts and let you know what needs to be done.

#5 Tire Belts Slipping

Last but definitely not least, we've got tire belt issues.

Modern tires are pretty amazing – they've got these internal belts that help them keep their shape and handle well. But over time, or if the tire gets damaged, these belts can slip or even start to separate.

When that happens, you'll probably notice a shaking or thumping, especially at certain speeds.

When tire belts slip, it usually feels like a rhythmic vibration that gets faster as you speed up. You might feel it through the steering wheel or the floor of the car, and it’s often worst around 40 mph.

This one’s a bit more serious, though. If the tire belt slips too much, it could cause the tire to fail suddenly, which is dangerous, especially at high speeds.

If you think your tire belts are slipping, get them checked out ASAP.

Usually, you'll need to replace the tire to stay safe.

Bottom Line

So there you have it – five common reasons why your car shakes at 40 MPH and then stops.  

Remember, while it's great to know what might be causing the problem, it's always best to get a professional to take a look. They know to diagnose and fix it properly.

Don't ignore that shake – it's your car's way of telling you something's not right. 

Get it checked out, and you'll be back to smooth cruising in no time.