Seventy-five heavy-duty public transit buses, including different fuel systems (conventional diesel, natural gas and diesel hybrid), were tested on-road in Beijing using portable emission measurement systems. Second-by-second driving condition data were collected on typical urban bus routes including freeways, arterial roads and residential roads. The average values of distance-specific fuel consumption for diesel buses is 32.6 L 100 km(-1) under a typical bus driving cycle in Beijing (BJBC). Natural gas buses have comparable CO2 emission factors but higher fuel consumption relative to diesel buses. Hybrid diesel buses are capable of reducing CO2 emissions and fuel consumption by 18-29% compared to the Euro IV and Euro V diesel buses over the BJBC. This study quantified the impacts on fuel consumption from other conditions including road type, average speed, load mass, and air conditioning. Average speed is the leading indicator of traffic conditions which affects the on-road fuel use most significantly. If the average bus speed decreases from 25 km h(-1) to 15 km h(-1), fuel consumption is estimated to increase by similar to 20-30% for diesel buses, similar to 30-45% for natural gas buses, and most significantly (similar to 50%) for hybrid diesel buses. In addition, real-world fuel consumption of hybrid diesel buses is observed to be particularly sensitive to operating conditions - when their on-board air conditioning systems are functioning, fuel consumption can be increased by up to 50%. (C) 2013 Elsevier Ltd. All rights reserved.