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.