Why won't car go over 40 mph?

Answer 1

I suppose there could be a transmission shifting problem. If you can't get to high gear you can't go fast.

Failing that, look for loss of engine output.

Loss of engine output, provided it runs smoothly at lower speeds, has only a few common causes.

1) Design limitation. Is the vehicle designed to go this fast? Some aren't.

2) Fuel limitation. From tank to carb/injectors are you getting enough fuel supply?

3) Air limitation. Do you have a clogged air filter? Damaged turbocharger? Worn cam? IMRC problem on a Ford?

4) Restricted exhaust system. This is usually easy to test for by unbolting the exhaust system immediately after the manifold(s) or header(s). If unbolting the exhaust makes car go fast, you need an exhaust system.

Other possible problems include overloading of vehicle or trailer, brake drag, or underinflated or incorrect size tires.