On preview I agree with KeithT but will post my little thought experiment anyway.
I hope nobody mentions the airplane and conveyer belt.
I do not have a factual answer in terms of having seen this done, and I haven’t seen it on TV except for a car commercial where it may have been special effects rather than real.
Assume you have a car driving 20 MPH approaching a truck going 10 MPH with a ramp dragging behind it. Assume the car is front-wheel drive. When the car’s front wheels hit the ramp, if they maintained the same speed then the car would drive up the ramp at 20 MPH relative to the truck. This is probably a bit fast for climbing a ramp onto a truck. However, the car is probably not going to maintain 20 MPH relative to the ramp for two reasons.
The car hitting the truck ramp is not a free ride for the car. Although the truck is pulling it at an added 10 MPH, the car has to hold on and pull itself along with the truck to overcome its inertia.
Let’s assume away a couple of things to simplify the model. Assume that the truck is a moving flat surface flush with the road, as though it were a moving piece of road. If you have ever been on an airport “people mover” you are very familiar with this idea. Think of the sensation you have when you first step onto the people mover. Although it is moving, it is not a totally free ride. When you first step on it, you have to push a little harder for a second if you want to keep walking. (Same thing in reverse when you step off; you have to decelerate.) When the car hits the truck ramp, the truck’s speed, faster than the road, requires the car to temporarily increase power to maintain 20 MPH relative to the truck (which is 30 MPH relative to the road). This assumes that the tires won’t slip under the extra power. Without the extra power surge, the car will not be able to “hold on” to the truck enough to maintain 20 MPH w.r.t. the truck, and the car will reduce its speed relative to the truck, as if it were going up a hill. However, it will still roll forward due to the fact that it was going 10 MPH faster than the truck. (If the car shifted into neutral when it hits the truck, then it would simply roll forward at 10 MPH w.r.t. to the truck, maintaining its original 20 MPH w.r.t. to the road.)
When you add the ramp back into the equation, the car has to climb uphill on the ramp, and that climb starts rather suddenly, unlike usual road conditions. The car would have to increase power instantaneously to maintain speed.
So the answer is 10 MPH or greater w.r.t. the truck depending on how much gas the driver gives it when reaching the ramp. If the driver applies brakes after reaching the ramp (probably a good idea unless he wants to drive right over the truck) then it will be less.
So the real answer is, “Any speed. It just depends.”