This paper describes a method for assessing the downstream milling energy requirements for high pressure grinding rolls (HPGR) products based on a Bond mill test procedure. Multiple trade-off studies have reported the performance of HPGR versus SAG milling with energy savinga of between 11 and 32 per cent. One factor that is often inconsistently defined in these studies is the change in the Bond Ball Work Index (BBWi). The Bond test can overestimate the reduction in ball milling energy requirements for HPGR products, not due to a change in the breakage characteristics of the particles, but because the Bond test feed for a sample crushed in a HPGR has a greater fines content than a conventionally-crushed sample. This paper rigorously assesses the actual change in BBWi achieved through the use of HPGR technology. The reduction in BBWi was found to not be dependent on the ore hardness and thus the expression of the change as a per cent is incorrect. When the size distribution of the HPGR product was matched to the crushed product, more than 95% of the samples tested resulted in a reduction in Work Index, with an average reduction of 1.9 kWh/t. A proportional reduction was seen when the Size Specific Energy (SSE) was calculated for the reconstituted samples. Six samples were tested where the original size distributions were retained and these saw greater reduction in BBWi than SSE. A gold mine was surveyed to investigate the ball milling requirements of an industrial HPGR circuit. Two surveys of the ball mill operating at different conditions found that milling efficiency could be improved by 20%. These results highlight the importance of the mills operating conditions on energy efficiency as they can overwhelm the potential benefits of HPGR pre-conditioning.