All your crew members each get the same amount of XP per battle, regardless of how many there are. So if your crew-training vehicle has more crew members, then you are effectively getting more total crew XP per battle. Also, some tanks are missing crew members, and their crew training efficiency is ridiculously low.
Tanks have an average of 4.3 crew members, and the tanks with the most have 6. These lists show the tanks that have extra crew members: displaying which they have more of, and which they don't have (if they're missing any). I've also pointed out which ones are Premium, as they give more crew experience. Note that "AT-SPG" is API terms for "Tank Destroyer".
73 tanks have 6 crew members. You'll notice that there are actually 11 tanks that have both two loaders AND two gunners, at the expense of a Radio Operator.
+gunner - 4
uk 2 lightTank Cruiser I Credits
usa 4 mediumTank M3 Lee Credits
usa 5 heavyTank T1 Heavy Credits
usa 6 heavyTank M6 Credits
+radioman - 8
usa 3 mediumTank M2 Medium Credits
uk 3 mediumTank Medium III Credits
ussr 4 mediumTank T-28 Credits
ussr 4 mediumTank T-28E F-30 Premium
japan 5 heavyTank O-I Exp. Credits
japan 6 heavyTank O-I Credits
japan 7 heavyTank O-Ni Credits
japan 8 heavyTank O-Ho Credits
+loader - 50
usa 3 SPG M7 Priest Credits
germany 3 mediumTank Gr.Tr. Premium
uk 3 SPG Sexton I Premium
uk 3 SPG Sexton II Credits
usa 4 SPG M37 Credits
germany 6 SPG Hummel Credits
ussr 6 heavyTank KV-2 Credits
ussr 6 heavyTank KV-2 (R) Premium
uk 6 AT-SPG AT 8 Credits
uk 6 heavyTank TOG II* Premium
usa 7 heavyTank T29 Credits
usa 7 SPG M12 Credits
ussr 7 heavyTank KV-3 Credits
uk 7 AT-SPG AT 15A Premium
uk 7 AT-SPG AT 7 Credits
usa 8 heavyTank T32 Credits
usa 8 heavyTank T34 Premium
usa 8 heavyTank T34 B Premium
usa 8 heavyTank T34 FL Premium
usa 8 SPG M40/M43 Credits
germany 8 AT-SPG Ferdinand Credits
germany 8 AT-SPG JgTig.8,8 cm Premium
germany 8 heavyTank Mauerbrecher Premium
germany 8 heavyTank VK 100.01 P Credits
germany 8 heavyTank VK 168.01 P Premium
germany 8 SPG G.W. Tiger P Credits
ussr 8 AT-SPG KV-4 KTTS Premium
ussr 8 heavyTank KV-4 Credits
ussr 8 heavyTank KV-4 Kresl. Premium
ussr 8 heavyTank KV-5 Premium
uk 8 AT-SPG AT 15 Credits
uk 8 AT-SPG Turtle I Premium
usa 9 AT-SPG T30 Credits
germany 9 AT-SPG Jagdtiger Credits
germany 9 AT-SPG WT auf Pz. IV Credits
germany 9 heavyTank Mäuschen Credits
germany 9 SPG G.W. Tiger Credits
ussr 9 SPG 212A Credits
uk 9 AT-SPG Tortoise Credits
france 9 SPG B-C 155 55 Credits
japan 9 heavyTank Type 4 Heavy Credits
usa 10 SPG T92 HMC Credits
germany 10 AT-SPG Grille 15 Credits
germany 10 AT-SPG Jg.Pz. E 100 Credits
germany 10 AT-SPG WT E 100 Credits
germany 10 heavyTank E 100 Credits
germany 10 heavyTank Maus Credits
germany 10 SPG G.W. E 100 Credits
uk 10 AT-SPG Badger Credits
japan 10 heavyTank Type 5 Heavy Credits
+gunner, +loader; -radioman - 11
uk 4 mediumTank Grant Credits
uk 4 SPG Birch Gun Credits
germany 5 AT-SPG Pz.Sfl. IVc Credits
ussr 6 AT-SPG SU-100Y Premium
ussr 6 SPG SU-8 Credits
ussr 7 SPG S-51 Credits
ussr 7 SPG SU-14-1 Credits
ussr 8 SPG SU-14-2 Credits
uk 8 SPG FV207 Credits
usa 9 SPG M53/M55 Credits
uk 10 SPG ConquerorGC Credits
43 tanks have 5 crew members, with a single duplicate crew member. These are still worth considering, because most of them are just at the expense of the Radio Operator, whose proficiency level literally just affects the signal range (also, whether you can communicate with an allied vehicle is determined by your combined signal ranges).
+gunner; -radioman - 2
uk 5 heavyTank Churchill I Credits
france 8 AT-SPG AMX AC 48 Credits
+radioman; -loader - 3
ussr 3 mediumTank T-29 Premium
japan 3 heavyTank Type 91 Credits
japan 4 heavyTank Type 95 Credits
+loader; -radioman - 38
germany 3 SPG Bison Credits
germany 3 SPG Wespe Credits
usa 4 AT-SPG T40 Credits
germany 4 SPG St.Pz. II Credits
ussr 4 SPG SU-5 Credits
france 4 SPG AMX 105 AM Credits
usa 6 SPG M44 Credits
uk 6 SPG FV304 Credits
france 6 SPG AMX 13 F3 Credits
usa 7 AT-SPG T28 HTC Premium
germany 7 SPG G.W. Panther Credits
ussr 7 AT-SPG ISU-122S Premium
ussr 7 AT-SPG SU-152 Credits
uk 7 AT-SPG Challenger Credits
uk 7 SPG Crusader SP Credits
usa 8 AT-SPG TS-5 Premium
ussr 8 AT-SPG ISU-152 Credits
ussr 8 AT-SPG SU-130PM Premium
ussr 8 AT-SPG T-103 Premium
ussr 8 heavyTank 703 II (122) Premium
ussr 8 heavyTank IS-2-II Credits
france 8 SPG Lorr. 155 51 Credits
usa 9 heavyTank M103 Credits
ussr 9 AT-SPG Obj. 263 Credits
ussr 9 AT-SPG Obj. 704 Credits
ussr 9 heavyTank IS-3-II Credits
ussr 9 heavyTank ST-I Credits
uk 9 SPG FV3805 Credits
usa 10 AT-SPG T110E3 Credits
usa 10 AT-SPG T110E4 Credits
ussr 10 AT-SPG Obj. 268 Credits
ussr 10 AT-SPG Obj. 268 4 Credits
ussr 10 heavyTank IS-7 Credits
ussr 10 heavyTank ST-II Credits
ussr 10 SPG Obj. 261 Credits
uk 10 AT-SPG FV215b 183 Premium
uk 10 AT-SPG FV4005 Credits
france 10 SPG B-C 155 58 Credits
In total:
73 vehicles have 6 crew
183 vehicles have 5 crew
284 vehicles have 4 crew
79 vehicles have 3 crew
28 vehicles have 2 crew
TL;DR – Some tanks get 3 times the crew XP as others.
Source: https://www.reddit.com/r/WorldofTanks/comments/f1q9uh/a_list_of_all_the_vehicles_with_extra_crew/
