I made a video to show you. The problem isn't that the Nissan is a road car, the problem is that the Nissan is an outlier that doesn't work properly within the PP system. I STRONGLY recommend watching the video. These cars are all set to "GR4" standard. The GR4 cars are all 650pp with Max downforce. The road cars are set to 650pp with Max downforce (because the amounts are about the same). The outlier is the Nissan that has to be 665pp to match the others. You can also see how the GR4 Scirocco and Suzuki, set the same way, simply can't match the other GR4 cars. (FYI, I throw it all away at the end, but you can see how close the race is throughout).
FYI Racing liveries = Racing car
@pabloprp69
To answer your question, throw all the racing parts at a car and be sure to include the rear diffuser. Then, slowly remove weight. How much depends on each car. Some only need stage 1 or stage 2. Always add the computer, the power restrictor, and the ballast so you can tune the power and weight.
For suspension, many cars default to being too low and too soft. Make the springs hard (Maximum or almost maximum) and given them 5-10 steps of extra ride height above the racing suspension default.
