Direct use of straight vegetable oil based biofuels in diesel engines without trans-esterification can deliver more carbon reductions compared to its counterpart biodiesel. However, the use of high blends of straight vegetable oils especially used cooking oil based fuels in diesel engines needs to ensure compatible fuel economy with PD (Petroleum Diesel) and satisfactory operational performance. There are two ways to use high blends of SVO (Straight Vegetable Oil) in diesel engines: fixed blending ratio feeding to the engine and variable blending ratio feeding to the engine. This paper employed the latter using an on-board blending system-Bioltec system, which is capable of heating the vegetable oils and feeding the engine with neat PD or different blends of vegetable oils depending on engine load and temperature. A used cooking oil derived SVO type of biofuel, the C2G Ultra Biofuel (C2G: Convert to Green), which is a fully renewable fuel made as a diesel replacement from processed used cooking oil, used directly in diesel engines specifically modified for this purpose, has been investigated in this research. A series of real world driving tests were conducted on a 44 ton articulated truck. A dual fuel tank containing both PD and the C2G Ultra Biofuel was installed. The engine was started with PD and then switched to C2G Ultra Biofuel gradually. The vehicle was tested on either neat PD or blended fuel mode with different load (empty or fully loaded trailer). The fuel consumption and tailpipe emissions were measured. This paper focused on the fuel consumption and GHG reductions. The fuel consumption was determined by volumetric, mass and energy per km travelled and per ton of GVW. The results show that the fuel consumption for neat PD and blends was at the similar level and has a good inverse linear correlation with GVW. The substitution ratio of PD by the C2G Ultra Biofuel is 86∼91% and 74∼81% for hot start and cold start trips respectively. The GHG reductions by the C2G Ultra Biofuel are 85∼89% and 73∼78% for hot start and cold start trips respectively.