Solved

Points or %

Which is better to improve the car ? 

For instance, I have 100 points in braking, and the best car has 140 (a difference of 40 points or 40% more). Alternatively, I have 156 points in handling, and the best car has 200 (a difference of 44 points or 28% more). Should I consider the point difference or the percentage increase? Focus on absolute or relative improvement ?

Thanks in advance !

Hello, you have to focus on the difference in points, the point gain in research depends on the points you get from the best car in the area you are researching.

The performance you get from adding points isn't linear, the gain tails off as it gets towards the maximum. Adding say 10 points on a low value would be the same as adding 30 on a higher one. With your values given, adding 10 on the 140 will reap a better gain than adding 10 on the 156. 
A balanced car I find performs better as each track has its own criteria for acc, bra, hand, down etc.