I don't know if you are using the components around VehicleBody or using a RigidBody or you even use a KineticBody and do all the physics calcs yourself.
Anyway, if you go with the game physics you'll have to watch which forces slow a car down and you then have to device which of these forces to actually implement. Which to ignore and which may be already implemented in the component you are using.
Speaking of VehicleBody my guess is that at least some friction (wheelsto ground) probably is already inside the component. Especially prominent when the turning direction of the wheels isn't identical to its movement direction over ground. (skidding)
However, there are two forces which you probably have to implement yourself. The most important one on higher speeds is air drag. Air drag will increase roughly in speed by square. So you could roughly use a formula like this to calculate the air drag:
drag = forceFactor dragCoeff Area (velocity velocity)
Where forceFactor is just a factor the scale the force in a linear way. You could think of it as the density of air.
The dragCoefficient is one factor related to how good the form of the car "cuts" into the air. When you want to be more accurate, you could vary this value depending on which side of the car is actually facing the "wind". (When the car is spinning or on side wind.)
Area is obviously the (projected) area of the car pointing into the air. This is big for a truck and small for a Renault Twizy. And the area can vary when the car is spinning or on side wind.
For a start you can mix up forceFactor, coefficient and area to one constant value depending on the vehicle type. That way you ignore the changing draft on skidding car but perhaps that is less important.
The direction of the drag force is (when the air is not moving = wind) just velocity * -1. Naturally this is also not 100% correct as the drag area might vary along the shape of the vehicle.
About delta:
You need delta when you want to scale some effect/movement/etc over time. An (repeatedly) applied impulse would have to be scaled by delta to sum up to the whole value over time. However, an applied force will just be applied or not applied. No need to scale that by delta IMHO. Just keep in mind that the force has to be applied repeatedly in every physics frame as Godot "forgets" it after each frame.
Oh, there's a special case: The vehicle "Cyclops" in the film The Big Bus breaks the "wind barrier" at 90mph. So if you're remodeling that vehicle then keep in mind that you set air drag to 0 on speeds higher than 90mph.
Another force which may considerably slow down a car is the Engine Braking:
Engine braking force will scale somehow on the rpms of the engine (shifting down to brake) and depends also on the type of engine (displacement, diesel/petrol). The force vector will be - equal to normal braking - along transform.basis.z. So you probably already implemented something like this.
