Interest in using unaltered vegetable oil as a fuel in diesel engines has experienced an increase due to uncertainty in the crude oil market supply and the detrimental effects petroleum fuels have on the environment. Unaltered vegetable oil blended with petroleum fuels is less expensive, uses less energy to produce and is more environmentally friendly compared to petroleum diesel or biodiesel. Here we investigate the engine performance of unaltered waste soybean oil blended with petroleum diesel and kerosene for three vehicles. Five biofuel blends ranging from 15% to 50% oil by volume were tested on a 2006 Jeep Liberty CRD, a 1999 Mercedes E300 and a 1984 Mercedes 300TD. A DynoJet 224x chassis dynamometer was used to test vehicle engine performance for horsepower and torque through a range of RPMs. Results for the Jeep showed a modest decrease in horsepower and torque compared to petroleum diesel ranging from 0.9% for the 15% oil blend to 5.0% lower for the 50% oil blend. However, a 30% oil blend showed statistically better performance (P < 0.05) compared to petroleum diesel. For the 1999 Mercedes, horsepower performance was 1.1% lower for the 15% oil blend to 6.4% lower for the 50% oil blend. Engine performance for a 30% blend was statistically the same (P < 0.05) compare to diesel. Finally, horsepower performance was 1.1% lower for the 15% oil blend to 4.7% lower for the 50% oil blend for the 1984 Mercedes. Overall, the performance on these oil blended fuels was excellent and, on average 1.1% lower than petroleum diesel for blends containing 40% or lower waste soybean oil content. The more significant decrease in power between the 40% and 50% oil blends indicates that oil content in these blended fuels should be no more than 40%. (c) 2013 Elsevier Ltd. All rights reserved.