How to add grip to achassis/how to fix the car that slips from anything

How can i fix my vehicles that don’t have grip?

i have some military jeeps that have already an a-chassis, but when i need to climb anything(also a little slope) the car takes very much time, so i want to know how i can increase the grip for all my vehicles

you can change the customphysicpropertis friction in the wheels(FL,FR,RL,RR)