Gas distribution networks will typically charge based upon the higher heating value of the gas (i.e. total energy content). A quoted engine efficiency based on LCV might be say 44% whereas the same engine might have an HCV of 39.6% based on HCV on natural gas. It is also important to ensure that efficiency comparisons are on a like for like basis. for example some manufactures have mechanically driven pumps whereas other use electrically driven pumps to drive engine cooling water, and the electrical usage can sometimes be ignored giving a falsely high apparent efficiency compared to the direct drive engines.