It must be possible for them to be fiddled, why else would the trading standards & revenue people need to check them if not ?
Sunny. Raining. Windy. A different type of riding, such as motorway or town. Making more haste on the tankful. Carrying pillion. Different tyre pressures. Different air temperatures. Different fuel temperatures when filling up. More red lights on the commute home. Wearing leathers or waterproofs. Wearing a rucksack.... I find it hard to believe that none of the above variables that could affect the mpg ever changes!
