The simple answer is most modders either haven't a clue what they're doing or don't care at all about getting data close to reality. The most common problems I've found (related purely to engines/aero):
- No transmission loss taken into account
- Engines just copied from a 'close enough' Kunos car
- Power/Torque/Weight figures changed in the UI only and not adjusted in data (seriously some people don't realise they're not one and the same)
- Power and torque figures entered without taking the turbo into account (which then gets added in on top)
- Aerodynamics that don't have any entries for drag so the top speed is only limited by gearing, this is a very noticeable issue on modern cars which have 7 or 8 gears as these are for economy, not top speed, yet without any drag they allow some cars to reach several hundred mph given enough room.
- Aerodynamics blindly copied from a random Kunos car (for some reason there's loads that have a ~97% similarity to the Zonda R)
- Issues with gearing/drivetrain/aero/tyres/suspension/weight that don't allow a car to reach the manufacturer claimed performance stats so get compensated for by adding more power.
For straight line performance I'd recommend this track
https://www.racedepartment.com/downloads/test-pad.10703/ with perfect conditions and no wind. Get your basic data as Mike173 mentioned, from
https://www.automobile-catalog.com/#gsc.tab=0 but beware that some of the data isn't totally accurate so go to a few different sources to compare if it's a car you really care about. Then for the aero, I start by copying the aero.ini and all the wing_****_CD.lut files from the most similar Kunos car, then adjust the size/positions in the CM showroom (plus add or subtract wings as needed). Test on the test track and open the aero app while you're making your top speed run, then make adjustments to the CD_GAIN and CL_GAIN figures in each wing of the aero.ini until you're happy with the downforce and the drag stops the car at the correct top speed (in cars not limited by gearing).