US artillery consumed an average of 1,500 tons of ammunition per day during July 1944. German records indicate that an average of 480 tons of ammunition of all types was sent by supply units to German combat forces including artillery units. This was the average amount dispatched not the amount delivered. Those records did not survive. Historian Niklas Zetterling estimates US artillery fired 4 times the amount of shells fired by German artillery.
(Source: Normandy 1944: German Military Organization, Combat Power and Organizational Effectiveness by Niklas Zetterling)