This issue has persisted since the first Forza Motorsport on the original Xbox. The R34 Skyline, together with around 50 other cars currently in Horizon 5, uses a deformed and low quality model taken straight from the first Forza Motorsport and just visually upgraded to look more on par with Xbox One models, but nothing was done about the body shape which is painfully wrong.
The front end of this car is very out of proportion, being both too small and having wrong shapes of everything. The headlights are closer to those of an Opel Astra from the same model year. The side line is inaccurate too, the roof is supposed to be more rounded, same with the side windows. Mirrors are completely wrong. The rear end is too narrow. The factory wheels are completely inaccurate, not being concave at all. Every part of the car has weird pre-shadowing artifacts. Every body kit the car currently has is also modeled wrong thanks to other inaccuracies, but the worst by far is the Nismo kit, where the front bumper is squished.
You should not add any new customization to this car until it recieves a new and accurate model.
