The broadest answer to this would be “just about everything!” However, there are some things that affect the car performance in a more significant way than others. Some of these make it easy to increase performance, some are more involved. There are times when it is a combination of things that adversely affect a car performance. Most often, people are looking for the simpler ways of improving their cars efficiency.
The carburetors function is to mix the air and fuel before sending it to the engine. If there are things that are “off” in the carburetor, it can easily cause a decrease in your engine performance.
The cooling system, aside from the radiator, include the water pump, head gasket, freeze plugs, hoses, thermostat, radiator fan, heater core and fan clutch. Something gone wrong in any or a combination of these components can adversely affect your car performance. More often than not, the trouble lies with a hose, or a fan, but not always. Having a problem in the cooling system can ultimately put an end to the life of your car. For this reason, any problem with the cooling system is one you should fix immediately.
This system is often overlooked; however, it can be a crucial part of your car performance. The job of the exhaust system is to clean the unhealthy fumes and gasses before emitting them out into the environment. The parts included in the exhaust system include the oxygen sensor, exhaust manifold, muffler, catalytic converter, and exhaust pipe. Again, a problem with any of these parts can hinder the performance of the entire system. While it is important for the life of your car, it is more important that damage to these parts can affect your health, and the health of others.
Wheel weight and size:
Most often, this is not an issue, unless you are customizing your car with wheels and special tires.
These are just some of the things that can affect the performance of your car. You should also keep in mind that the fuel you choose also can affect your car performance. How often you fill your tank can also affect performance. One thing that few people consider is the weight inside their car, and how it adversely affects the car performance. This includes extra body weight, and cargo weight.
Never forget to check the oil and to make sure there are no leaks. Problems with lubrication can end the life of your car. Find out which oil is best, have the system checked, and always check the oil level.
While there is not a lot that can be done about this, it is obvious to most people that the larger the car, or SUV, the more it affects the overall performance of the vehicle.
In case of concern, always have your car checked by a professional. Leaks, and hoses are easy to replace, but the entire car is not!