Wow, as much as i can understand the initial confusion, i don't see what's
so unlogical about it. Quite the contrary. Each car has a "platform" name
(often referring to the engine/drivetrain) and some add a "model name".
If you start from the beginning, it's very simple. There was only had 2 cars: 
The XF and the XR. The xF is Fwd while the xR is Rwd. That's the logic. 
 
Then S1 brought a new wave of platforms, the RB, FX and the LX. The 
RB4 is 4wd (it's acually an AWD system, but that's a detail, all 4 wheels
are driven anyways, hence the 4). This is where it gets more confusing. 
FXO has that 'O' (often unclear if it's '0' or 'O'..) which has no meaning
or at least it's not obvious. As for the LX twins, well there's the LX4 with
a, you guessed it, 4 cylinder engine while the LX6 has a 6 cylinder engine.
The MRT5 is a real car who's real name IS the MRT5 so that one i'll put aside.
 
Finally, there's S2 with the UF, FZ and Formula cars. Again, UF is the car's
platform, in this case it's a Mini-Clone, in UF1000 version, it contains a, 
surprise, 1000cc engine. The FZ50 imo is an acronym made to show the cars
that inspired it, in this case the 50 could refer to the Ferrari 550 Maranello 
and/or the Nissan 350Z, notice the Z there...The car DOES have a 350Z 
front with a 550maranello rear after all 

 Anyways, that name is open for 
speculation, i doubt the devs would confirm that one of their cars is inspired 
by a real car...Just to add some oil on the fire, the drivetrain is pure Porsche 
911, a 3.6L flat 6 positioned behind the rear axle. Then there's the Formula 
cars. Personally, i find those obvious. The Formula XR is a small formula car 
with a XR engine, a 2.0L inline 4. Basically a 'street' engine that 
manufacturers showcase. The Formula V8 is the bigger brother with, tadam,
a V8 engine. The RA (RunAbout) is also a real car whoe real name IS the RA.
 
There isn't a specific logic to every single letter, just like Nissan uses 'X' all 
over it's cars for no reason or significance. It's just a cool letter to have on 
a car i guess. However, most digits refer to the specs of the car, like it's 
engine or drivetrain. Now for the acronyms. LFS uses 3 letter acronyms for 
one main reason i think, there's only 4 caracters in a byte ! That said, there is
room for one more caracter, but it's often kept free or contains other info.
As the names got long and hard to scan in a list, the acronyms are very 
usefull in classifying all the cars quickly. Notice it's main use is online where
the less bytes you transfer, the better. 
 
For the GTRs, you take the first two caracters of the platform name and
add a 'R' at the end (ie: XF GTR version is XFR). The Formula cars acronyms
are very simple, it's 'Fo' for 'Formula', then add the 'X' for 'XR' or '8' for 'V8', 
so you get FoX or Fo8.
 
Sooo, in order we have:
 
Platform===Models==========Acronym
 
XF -------- XF GTi------------ XFG
----------- XF GTR----------- XFR
 
XR -------- XR GT------------ XRG
----------- XR GT Turbo------ XRT 
----------- XR GTR----------- XRR
 
RB -------- RB4 GT ----------- RB4
 
FX -------- FXO Turbo ------- FXO
----------- FXO GTR --------- FXR 
 
LX -------- LX4 -------------- LX4
----------- LX6 -------------- LX6
 
MRT ------ MRT5 ------------ MRT
 
UF -------- UF1000 ---------- UF1
----------- UF GTR ---------- UFR 
 
RA -------- RA -------------- RAC 
 
FZ -------- FZ50 ------------ FZ5
----------- FZ GTR ---------- FZR
 
Formula --- Formula XR ------ FoX 
----------- Formula V8 ------ Fo8