The effect of dehydrated ethanol and rapeseed methyl ester additives to diesel fuel on engine fuel consumption has been experimentally investigated. The used average capacity (18 kW) tractor was driven by unmodified direct injection (combustion chamber consists of a dished piston) four cylinder, two-cycle diesel engine D21A1. Multicomponent fuel blends were prepared by adding 5 and 10% (v/v) of ethanol as well as 30 and 50% (v/v) of biodiesel (RME) to diesel fuel obtained from Mazeikiu Nafta Petroleum Refinery Company (Lithuania). During the on-farm tests tractor's fuel consumption data range varied at the margin of error depending on the used type of fuel. In order to solve this problem the decision was taken to attempt to find the optimum number of test drives by using of statistical modeling on the basis of the mathematical theory of experiment. The use of this method is advised when there is a large number of interdependent factors which influence the final result.