I'm of the belief that sound is as important as anything in creating immersion - and I've found some of the cars' sounds ingame don't quite match the experienced RPM.
While I'm very impressed with the game's sound design (especially with the recent 2024 DLC and additional content packs) I think there's a few things that could be done to several cars to make it even better.
I've created this spreadsheet to better illustrate the difference between the frequency the sounds ingame play at (as of v2.3.1) and the frequency they should be playing at; hopefully these mismatches can be rectified in future updates. Below I've detailed some of the most important ones in my view:
H2 (FWD)
Peugeot 309: For some reason the Peugeot 309 ingame sounds like the engine is only achieving about 6000 rpm at the 7000rpm redline - which is very strange considering it uses the same engine as the 205, which sounds almost perfect already! This video illustrates the issue (although the 309 in this video does rev a little higher than the 7000 of the ingame cars, it's still clear that the pitch is a bit off ingame right now).
H2 (RWD)
Talbot Sunbeam Lotus: While the ingame car revs to about 8000 rpm, it only sounds like it's doing about 7000 - here we can see a Group 2 car (like the one we have ingame) revving to ~ 7000 rpm, and the pitch is the same as the simulated vehicle despite the latter having a 1000rpm higher redline.
H3 (RWD)
Renault 5 Turbo: at the redline of 8000 rpm, the car sounds like it's revving to 7000 rpm - which is 1000 rpm lower than it really is.
Group B (RWD)
Lancia 037 Evo 2: at the redline of 8500rpm, the car sounds like it's revving to 7375 rpm - which is 1125 rpm lower than it really is.
Opel Manta 400: at the redline of 8000 rpm, the car sounds like it's revving to 7500 rpm - which is 500 rpm lower than it really is. (Same issue as the Opel Ascona above)
Group B (4WD)
MG Metro 6R4: The car's tachometer indicates a redline of about 9750 rpm; however at the redline the engine sounds like it's not breaking the 9000 rpm barrier, rather stopping at 8750. This video illustrates the issue brilliantly - as the engine RPM approaches 9000 on the IRL tachometer, the pitch is the same as the ingame car at 9750 - meaning the virtual MG's engine audio is about 1000 rpm too low at the redline.
Group A
Mitsubishi Galant VR4: For some reason, the engine pitch rises inconsistently with RPM after about 4250rpm. Up to this point, the pitch is correct - but at the 8000 rpm redline, the engine audio is the same pitch as that of the 7000rpm Peugeot 205. The pitch from 4250rpm+ needs to scale linearly up to 8000rpm.
F2 Kit Car
Citroën Xsara Kit Car - Telemetry indicates 9,750 rpm redline for this car. I believe this may be a physics error, as I have not seen any footage of Xsaras running at 9,750 rpm - and I believe the sound was recorded with a 9000 rpm redline - but if this is not the case, the sound is about 750 rpm too low for the redline.
Peugeot 306 Maxi - Since this car has no MoTeC display to show RPM, I've used the UDP telemetry to find the redline; it indicates 11,000 rpm redline for this car. I believe this may be a physics error, as I have not seen any footage of 306 Maxis running at 11,000 rpm - and I believe the sound was recorded with a 9750rpm redline - but if this is not the case, the sound is about 1500rpm too low for the redline.
Volkswagen Golf IV - The LCD display indicates a redline of 9000 rpm; the same as the Seat Ibiza, however the engine pitch at this redline is lower than the Spanish car; the Golf's audio is about 750 rpm too low at the redline. (This video illustrates correct pitch.)
WRC 1997-2011
Citroen Xsara WRC: The LCD display on the Xsara shows it has a redline of about 6500 rpm - but the engine audio at this redline is the same pitch as the Citroen C4's, which has a redline of about 7500 rpm. (This may be due to both the Xsara and C4 using the same sound banks; the C4's engine audio is perfectly synced to its rev range.)
Ford Focus RS 1999 & 2001: The display on both cars indicates a redline of 7200 rpm; which is almost the same as the 2008 car - however the engine pitch at this redline is much higher for the older cars, about 600rpm too high. The 2008 car's audio is accurate to the RPM experienced.
Mitsubishi Lancer Evo VI: While the ingame car revs to 7000 rpm, it sounds like it's doing 8000. This video of Tommi Mäkinen at the 1999 Monte Carlo rally shows gearchanges at the 6000-6500 mark - as is the supposed optimum ingame - but the sound at this point is equal in pitch to the ingame vehicle at just 5000-5500 rpm - meaning the sim's represenation sounds like it's revving 1000 rpm higher than it really is. Furthermore, the engine pitch rises inconsistently with RPM after about 3750rpm (the same issue as the Galant above; passibly as the two cars share engine audio banks).
NR4/R4
Mitsubishi Lancer Evo X: at the redline of 7500 rpm, the car sounds like it's revving to 8000 rpm - which is 500 rpm higher than it really is. Furthermore, the engine pitch rises inconsistently with RPM after about 3750rpm (the same issue as the Galant above; possibly as the two cars share engine audio banks).
Rally4
Peugeot 208 Rally4: While the ingame car revs to 6750 rpm, it sounds like it's doing about 7500 - in this clip we can see a 208 shifting at about 5750 rpm, but the sound is equal in pitch to ingame vehicle at 5000 rpm - meaning the sim's represenation sounds like it's revving 750 rpm higher than it really is.
