A couple of things
BlauK - The 8th AF was the only US unit that gave credit for ground kills, but this was only during the war. None of the other AF's did this. They did this to "encourage" pilots to straffe the LW on the ground (I believe Gabreski said he'd rather face a hundred 190's than straffe) However, after the war, the USAF Claims Board went through all kills claimed by USAAF pilots and removed the ground kills claimed by 8th AF pilots. Therefore when you see an 8th AF pilot's kill credits, for instance Col John Meyer is credited with his 24 air to air kills, you know that is how many he scored air to air. If his ground kills were still counted, his score would be 37.
AFA the unit sizes are considered. The normal USAAF fighter group was made up of three fighter squadrons of 12-16 planes for a total number of 38-48 aircraft. Compare that to a jadgeschwader which could have anywhere from 90-120 planes at full strength. Rougly 2 to 3 times the size. So if you really wanted to compare kill totals you'd compare I/JG 54 to the 352nd FG (for instance)
For relative unit sizes; I took these numbers from Spades' excellent
12 O'Clock High website.
Formation - USAAC/F - RAF - Luftwaffe - No. a/c (US)
Administrative - Wing - Group - Geschwader = 5 groups
Operational - Group - Wing - Gruppe = 3 squadrons
Operational - Squadron - Squadron - Staffel = 16-24-36 a/c
Tactical - Section - Section - n/a = 8 a/c
Tactical - Flight - Flight - Schwarm = 4 a/c
Tactical - Element - n/a - Rotte = 2 a/c
So you can see a JG is much bigger than a FG.
Some of the USAAF's highest scoring FG's were the:
49th - 678
31st - 570
354th - 701 (first unit in ETO to fly P-51B's)
475th - 552
The five highest scoring 8th AF FG's were (in terms of air to air kills)
56th - 674 (128 losses)
357th - 609 (128)
4th - 583 (241)
352nd - 519 (118)
355th - 365 (175)
However, to get a true comparison, you'd have to compare the scores of a whole wing to those of a Jadgeschwader. The 8th AF was made up of 3 Fighter Wings, the 65th, 66th and 67th. The 67th Fighter Wing was made up of the 20th, 352nd, 356th, 359th, and 364th FG's. Their combined air to air victories equal 1441 kills. The 65th Fighter Wing (4th, 56th, 355th, 361st and 479th) scored 2003 kills while the 66th FW (55th, 78th, 339th, 353rd, 357th) scored 1832 kills.
Of course all of this must also be weighed against things like when the FG's went into service:
4th - 10 Mar 43
20th - 28 Dec 43
55th - 15 Oct 43
56th - 3 Apr 43
78th - 12 Apr 43
339th - 30 Apr 44
352nd - 9 Spt 43
353rd - 12 Aug 43
355th - 14 Apt 43
356th - 15 Oct 43
357th - 11 Feb 44
359th - 13 Dec 43
361st - 21 Jan 44
364th - 3 Mar 44
479th - 26 May 44
So I think it'd be fair to say that the LW units got a head start

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Col Dune
C.O.
352nd Fighter Group"The Blue Nosed Bastards of Bodney" "Credo quia absurdum est." (I believe it because it is unreasonable)
- The motto of the Republic of Baja Arizona
[This message has been edited by Dune (edited 03-29-2001).]