A modified car consist of any of the components associated with car tuning, however common upgrades include replacing the standard factory issue wheels with aftermarket alloy wheels, usually of a larger diameter to improve the aesthetics; lowering the suspension of the car to give it a more aerodynamic, stable look by replacing the springs (often in conjunction with shock absorbers) or torsion bars; replacing the existing exhaust system with one with less gas-flow restriction, louder and deeper sound and often small improvements in performance (due to exhaust gas-flow rate). Other common upgrades include the installation of a short shifter as well as an aftermarket shift knob, or a performance oriented Weighted Gear Knob. Common non-mechanical upgrades include a cars audio system; though not performance oriented, are nevertheless a very common-place modification. This includes upgrading the head unit itself, and upgrading and/or installing additional speakers.

Significant modification to a car could involve adding a fiberglass body kit to improve the aesthetics, a complete respray to a more unusual, distinctive and noticeable colour and finish, increasing the engine performance with ugraded components such as a high-flow air filter, additional components such as turbocharger, supercharger, Nitrous oxide kits, aftermarket cylinder heads or porting and polishing of O.E.M. heads which require retuning of electronics (later model vehicles - fuel injected) and modifications to vehicle sub-frames to accommodate additional horsepower caused by said changes. Traditional customizing methods, more common when modifying classic cars to create cars such as Hot rods, include roof chops, two door stretches, suicide & lambo doors, channeling and lights, trim and sheet metal from another car. Sectioning is also used sometimes.

The essence of modification of a tuner car is an attempt to extract the greatest possible performance—or the appearance of high performance—from the base motor vehicle through the addition, alteration or outright replacement of parts. Although this largely involves modifying the engine and management systems of the vehicle to increase the power output, additional changes are often required to allow the vehicle to handle this power, including stiffened suspension, widened tires, better brakes, improved steering and transmission modifications such as the installation of a short shifter. Although largely invisible from outside the vehicle, certain modifications such as low profile tires, altered suspension, and the addition of spoilers can change the overall appearance of the car, As well as adding downforce to increase traction.

Engine tuning is the process of modifying the operating characteristics of an engine. In a typical engine set-up, there are various mechanical and electronic elements such as the intake manifold, spark plugs,Mass air flow/ Volume air flow, etc. Modern engines employ the use of an ECM to provide the best balance between performance and emissions. Via the OBD communications protocol, the electronically controlled aspects of the engine can be modified in the process known as 'mapping'. Mapping can either be performed by changing the software within the ECU (chip tuning via firmware modification), or by providing false data via plug-in hardware (piggybacking). Mechanical components can also be replaced, such as turbochargers or superchargers.

Other standalone engine management systems are available. These systems replace the factory computer with one that is user programmable.

Poorly executed modifications can have a detrimental effect on performance as well as mechanical and electronic components. An example would be the use of an air compressor such as a turbocharger to increase the volume of air used in power stroke of the otto cycle. In a typical chemical reaction, the air-fuel ratio must be a minimum of 14:1 (see Stoichiometry). If higher ratios are used, higher pressures and temperatures are observed in the cylinders. Depending upon the build of the engine, such operating parameters can lead to premature failure such as warped cylinder heads and walls (temperature related), cracked connecting rods and crankshafts (excessive torque applied), engine seizing and blowouts.