The numbers I saw suggested that the Brits managed a 'tooth to tail' ratio of about 1:1. Why was it possible for them and not the US? Or am I comparing apples and oranges?
I strongly there is a variable in how the civilian or non uniformed support is counted. A lot of support manpower of the German military came in the form of the Todt organization, who are ususally not counted because they were civilians. This was a similar if perhaps lesser factor with the British. In the case of the US it was difficult to send civilian labor overseas. The USN used civilian contractors for their construction work in and outside the US, until circumstances made the conversion of civilian labor to Naval personnel in the Construction Battalions (Sea Bees). Port Operations Units were another group of naval personnel who under many other circumstances would have been independant civilian contractors with only a few naval officers as liasion or supervisors. Most of the heavy construction battalions and many of the medium of the US Army were similar.
There were US Army or Navy support units working with Allies combat units as well. ie: the Chinese Corps that fought in Burma with US Army weapons and supply also had a number of US Army support units sent over in 1942 to 1944 to provide logistics support. Ditto for the Free French Army from when it was requiped and supplied by US sources from mid 1943, ie: The USAAF airfield construction and support units sent to Corsica in November/December 1943 helped set up the air base for the French air groups as well as the USAAF.