A rival tends to brake more than necessary before a corner, although they are fast in a straight line. Therefore, even if you have a car with poor performance, there is still a high chance of winning a battle if you battle in a place with many corners.

You can asses the strength of rivals by looking at their car model and seeing what team they race for is.
Even if you lose, you still gain a little bit of CP so you can battle a rival to find out their strength. You might be surprised and find some rivals are slower than you anticipated.