Search for Garage editor it cant save the cars but you can save the settings and each time you start the game you can inject the garage
Sorry, but this is not what I am looking for. I do not want to change my cars, I want to change the value/stats of the cars temporarily (speed, handling etc.)
This is possible with a Trainer available here on the forums. It works by setting vehicle gravity higher this gives them better handling and increased speed / acceleration very useful for races!
Thread = GTAV Online Trainer By Felix The Furry v1.3.5
(Sorry cannot post links)
You cannot directly edit the values permanently but this trainer will help you to get the unlocks required to make the car faster and have better handling