Low total cost of power is the principal aim of every farmer owning a tractor. His fuel problem is one of selecting from the common fuels available one that is best suited for his particular tractor and conditions. Convenience and increased flexibility represent intangible values, but they must be considered in the overall tractor power cost. Recent improvements in tractors such as electric starters, rubber tires, and higher ground travel speeds have increased the interest in fuels which permit greater flexibility and higher performance. In times of high prices for farm products, farmers generally show a greater interest in high-quality, high-performance fuels. In times of low prices the cash outlay for fuel is given greater consideration, and performance and flexibility take a less important position. Tractor engines are peculiar in that they are the only large class of engines designed to use a wide variety of fuels. Tractor engines on farms today vary in their fuel requirements and will operate on fuels ranging from zero octane on up to premium grade gasolines. As the situation stands, there are available to the farmer, tractor engines that will utilize high-priced gasoline with high power and efficiency but are unable to burn low-grade fuels, and also a type of engine that will burn either cheap, low-grade fuels or gasoline with relatively low power and efficiency. It might appear without having given the problem sufficient thought that there is no justification for the existence of this complex situation. One might take the position that all tractors should be high compression and thus follow the trend of car, truck, and airplane engines. A brief study of the facts, however, will show why tractor engines have been, and still are built which will operate on fuel of wide quality range. This situation does not simplify the problem for the farmer by any means, nor is it always the most efficient in the use of our fuel resources. Table I shows some of the common physical properties of fuels being used in tractors. This represents the range that might properly be used by farmers in tractors in some part or other of the United States.