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Fuel-saving devices are sold on the aftermarket with claims they may improve the fuel economy, the exhaust emissions, or optimize ignition, air flow, or fuel flow of automobiles in some way. An early example of such a device sold with difficult-to-justify claims is the 200 mpg‑US (1.2 L/100 km) carburetor designed by Canadian inventor Charles Nelson Pogue.
The US EPA is required by Section 511 of the Motor Vehicle Information and Cost Savings Act to test many of these devices and to provide public reports on their efficacy; the agency finds most devices do not improve fuel economy to any measurable degree, unlike forced induction, water injection (engine), intercooling and other fuel economy devices which have been long proven. Tests by Popular Mechanics magazine also found unproven types of devices yield no measurable improvements in fuel consumption or power, and in some cases actually decrease both power and fuel economy.Other organizations generally considered reputable, such as the American Automobile Association and Consumer Reports have performed studies with the same result.One reason that ineffective fuel-saving gadgets are popular is the difficulty of accurately measuring small changes in the fuel economy of a vehicle. This is because of the high level of variance in the fuel consumption of a vehicle under normal driving conditions. Due to selective perception and confirmation bias, the buyer of a device can perceive an improvement where none actually exists. Also, observer-expectancy effect can result in a user subconsciously altering driving habits. These biases can be either positive or negative to the device tested, depending on the biases of the individual. For these reasons, regulatory bodies have developed standardized drive cycles for consistent, accurate testing of vehicle fuel consumption. Where fuel economy does improve after the fitment of a device, it is usually due to the tune-up procedure that is conducted as part of the installation. In older systems with distributor ignitions, device manufacturers would specify timing advance beyond that recommended by the manufacturer, which by itself could boost fuel economy while potentially increasing emissions of some combustion products, at the risk of possible engine damage.

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