The early P-38's had a range (on 300 US gal internal) of ~900 mi with ~1,750 mi with 2x 165 US gal drop tanks.
They had problem though, the lack of cockpit heating heating could be a considerable hinderance for the pilot at high altitudes (particularly in the cold skies of Norther Europe). There were some engine problems (possibly fuel or maintenence related, sometimes overcooling of the radiators too) and problems with the turbochargers at high alt. The intercoolers in the wing LE were inadequate to allow full combat power at altitude (designed for 1,150 hp). And of course there were the compressibility issues.
All models (prior to the boosted ailerons on the late P-38J's) suffered from mediocre roll performance, and prior to the addition of maneuvering flaps on the P-38G also had rather poor turn rate. (the flaps, combined with an increased in engine power gave the P-38G roughle double the sustained turn rate of the P-38F)
The P-38 was the best climbing fighter the US had at the time though, and was slightly longer ranged than the contemporary P-47's. (and significantly longer ranged than the P-47's w/out wing tank capability)
The P-47 pre-D-25 (with 305 US gal) had ranges of ~800-850 mi clean, and the max extenal fuel carried on the belly was a 200 long US gal semi-conformal laminated paper tank that was not well liked. And even then it would only allow ~1,400 mi max range.
Once they started getting wing pylons standard (some were modified in the field, but it was a long and time consuming process, particularly with the necessary plumbing for wing tanks) the 200 gal ferry tank was not used at all. (though a 200 gal flat steel belly tank was introduced later in the war) With the wing pylons they could carry 150 US gal under each wing plus one 75 or 110 gal belly tank. This allowed a max "yardstick" range of ~1,850 mi with 410 gal external, and a max combat radius of ~600 mi. (which would allow a maximum of only ~15 min at combat power with 30 min reserve at cruise power -assuming tanks are dropped at start of combat-)
I believe the early P-38's had nearly double that combat time at similar ranges.
The first P-38Js' range decreased a bit as they still had only 300 US gal capasity (and it was a bit less fuel efficient than it's predicessors, due in part to the increased drag of the new "chin" intercooler/oil cooler intakes), but this was quickly mitigated by a 62 gal tank added to the LE of each wing.(where the intercoolers had prevously been) This increased internal capacity to 424 US gallons and max range (on internal fuel) to ~1,200 mi.
Starting with the P-47D-25 internal capacity was increased by 65 gal to 370 US gal allowing a max range of 1,050 mi on internal fuel. But this model was introduced after the P-51 began taking over escort duties.
By comparison the ealr P-51's (B's and C's w/out fuse tank) ranged ~950 mi on internal fuel. (with fuse tank ~1,350 mi, though part of the fuse tank had to be used up before being stable for combat)
Also range figures for models with the V-1650-7 seem to be significantly lower (over 100 mi) than those with the 1650-3.
http://www.wwiiaircraftperformance.org/mustang/p-51-tactical-chart.jpg
