My hypothesis why the Mazda Roadster TC was added to the N-series is because the "staple" groups are relatively uniform in power & weight, even before BoP. Whereas while the TCs/RMs from prior games are indeed enhanced versions of the respective road cars, their upgrades don't lead to all TCs/RMs being relatively similar in power & weight.
According to
this promotional video, the classes are further defined as per the following descriptions:
Gr.4 - "Relatively subtle aero parts and weight reductions."
Gr.3 - 1350kg, 550-600HP
Gr.1 - 900kg, ~1000HP
Gr.B - 4WD, 500HP or more.
However, this video is from the launch window, before the introduction of Gr.2. We also should consider that the vision of a game and its scope could change over time. In my personal glance at the data, here are the current ranges of each class in terms of power & weight. Keep in mind that these metrics are
before BoP applications:
Gr.4 - 2105-3638kg, 296-436HP, comparatively light modifications.
Gr.3 - 2568-2976kg, 495-678HP, FR/MR only
Gr.1 - 830-1250kg, 501-939HP, various Sports Prototypes.
Gr.B - 1000-1380kg, 500-589HP, 4WD only
Gr.2 (not available at time of video's release) - 1020-1150kg, 487-614HP, FR/MR only, extraordinary aero kits.
There are some exceptions where cars will be outside these ranges, but not by much. For example, the Ford GT LM Spec II Test Car is only about 1165kg, and the Mercedes-Benz AMG-GT3 is a little above the 600HP mark that's mentioned in the promo video, at 613HP. Perhaps these metrics mentioned in the video are the intended stats for after BoP is applied. Which makes me wonder: would it end up being more fair if every car in a class had an almost uniform power and weight after BoP is applied?
That said, with all this in mind, I think we can use these stats to get an idea of what cars would go into these "staple" groups, both from prior GT games as well as real life. Going by GT6's car list, here are my conclusive picks for which cars would be in which class if they came to GTS:
Gr.4: Alfa Romeo 155TI, BMW 320i Touring Car, Jaguar XJ220 LM Race Car, Lexus IS-F CCS-R, Lexus IS-F Touring Car, Mazda RX-7 Touring Car, Mercedes-Benz 190E Evo II Race Car, Mitsubishi Lancer Evo IX Touring Car, Nissan Calsonic Skyline GT-R R32, Nissan Falken GT-R R34, Nissan GT-R R35 Touring Car, Nissan Skyline GT-R R32, R33, and R34 Touring Cars, Nissan Silvia S15 Touring Car, Pagani Zonda LM Race Car, Subaru Impreza Touring Car, Toyota Altezza Touring Car
Gr.3: Acura NSX LM Race Car, Audi R8 LMS - Team Oreca, Audi R8 LMS Ultra, R8 (LMP which was the road car's namesake), Blitz Dunlop ER34 Skyline, BMW M3 GTR Race Car, Chevrolet Camaro Mk.IV Race Car, Chevrolet Corvette C5-R, Chevrolet Corvette (C6) Z06 Race Car (also the ZR1 version), Ford Falcon XR8, Lister Storm Race Car, Mazda RX-7 LM Race Car, Mitsubishi FTO Super Touring Car, Lamborghini Nomad Diablo GT-1, Mercedes-Benz CLK Touring Car, Subaru Impreza Super Touring Car
Gr.1: Pretty much all cars from Group C, LMP900, LMGTP (such as the TS020), and LMP1. Not sure about that Zytek LMP2 car, though, nor the Toyota 7. The Chaparral cars, of course, would likely be put into Gr.X, though the 2D could find a home in a historic sports prototype class if it were made, perhaps alongside the Ferrari 330 P4, Jaguar XJ13, and Ford Mark IV.
Gr.B: Audi Quattro S1 Rally Car, maybe the other Group B cars, but I'm not sure.
Gr.2: [Pretty much every prior GT500 car from both JGTC and Super GT], Lexus IS-F Racing Concept, [many, if not all the GT1-spec cars from after the BPR era, as well as the various DTMasters cars which are not to be confused with the
other DTM, being DTMeisterschaft],
Keep in mind that in spite of having some classes feature both modern and older race cars, I think this potential issue would be greatly mitigated if there were more events where only some cars within a class could be used, like the recent Gr.4 events that only permit the FF cars within that class, or the N300 races which featured the "Gentleman's Agreement" JDM cars.
Other interesting notes:
-While the GT300 cars like the Autobacs Arta Garaiya or Weds Sport IS350 seem like they'd be a good fit for Gr.3, they have notably less power than the rest of the Gr.3 cars, including the made-for-game Lancer Evo X Gr.3 and WRX STI Gr.3, which both resemble GT300 cars.
-Similarly, while a lot of the real-world Group B cars seem like a shoe-in for Gr.B, their power and weight metrics seem quite lower than the rest of the cars currently in that class.
-As an aside, I still don't see why the BMW M3 (E92) GT isn't called the M3 GT2, unless that was its title within BMW.
-Some of the hillclimb cars may have too much power to be in Gr.B, like the Suzuki Escudo.