Biodiesel may be derived from either plant or animal sources, and is usually employed as a compression ignition fuel in a blend with petroleum diesel (PD). Emissions differences between vehicles operated on biodiesel blends and on diesel have been published previously, but data do not cover the latest engine technologies. Prior studies have shown that biodiesel offers advantages in reducing particulate matter, with either no advantage or a slight disadvantage for oxides of nitrogen emissions. This paper describes a recent study on the emissions impact of two biodiesel blends B20A, made from 20% animal fat (tallow) biodiesel and 80% PD, and B20B, obtained from 20% soybean biodiesel and 80% PD. These blends used the same PD fuel for blending and were contrasted with the same PD fuel as a reference. The research was conducted on a 2007 medium heavy-duty diesel truck (MHDDT), with an engine equipped with Exhaust Gas Recirculation (EGR) and a Diesel Particulate Filter (DPF). The truck's emissions were characterized under steady state conditions under three driving modes namely Mode1 (MD1), Mode2 (MD2) and Mode3 (MD3) using the West Virginia University's Transportable Heavy-Duty Chassis Dynamometer Emissions Testing Laboratory. For MD1, the vehicle was operated at a steady speed of 20 mph for 30 minutes. Similarly, for MD2 and MD3, the vehicle was operated at 35 mph for 30 minutes and 50 mph for 20 minutes respectively. Emissions were measured using a full exhaust dilution tunnel equipped with a subsonic venturi and secondary dilution for particulate matter (PM) sampling. The study showed that emissions are dependent on fuel types and vehicle speed. For any given cycle, the carbon dioxide (CO₂) emissions at the tailpipe produced by the PD, B20A and B20Bwere generally at the same level with a variation of between 0.5% - 1.4%. The results further showed that, at 35 mph, CO₂ emissions produced was lowest with corresponding highest fuel economy (miles per gallon of fuel consumed). The NOx emissions produced for B20A and B20B were slightly higher than those of PD, except for MD2. Generally, low particulate matter (PM) emissions were produced from the test results due to the truck DPF. The carbon monoxide (CO) and hydrocarbon (HC) emissions were also low, with HC being difficult to quantify as a result of oxidation in the DPF.