Normally, at high engine speeds the accessories consume unnecessary power. This can be counteracted by an accessory drive with variable speed ratio that reduces the speed of the accessories at high engine speeds.Simulations of the fuel consumption and acceleration performance of heavy vehicles with a variable-ratio accessory drive have been carried out. A 7% reduction potential was found for the fuel consumption of a city bus with operating air conditioner. This potential was lower without air conditioner; about 3%. The acceleration performance was improved similarly. For buses at highway driving the results were similar. However, for trucks the corresponding potential was only about 1%.Two variable-ratio accessory drives were studied. In general, a continuously variable design gave better results than a two-speed stepped device.