What you want to do is keep the angle between your direction of motion and the line of sight to police car the same as the angle between the truck's direction of motion and the truck's line of sight to the police car.
In other words, we want to keep $a1=a2$ in the picture above.
This is a problem in similar triangles. The answer will be that the ratio of your motion to your distance to the cop will have to be the same as the ratio of the truck's motion to the truck's distance to the cop. This can be seen from the following:
We note that $$\cot{(a1)}=\frac{\textrm{Truck's Speed}}{d1}$$
$$\cot{(a2)}=\frac{\textrm{Car's Speed}}{d2}$$
Set $a1=a2$ so we are always hidden behind the truck and solve for $\textrm{Car's Speed}$:
$$a1=a2$$
$$\cot{(a1)}=\cot{(a2)}$$
$$\frac{\textrm{Truck's Speed}}{d1}=\frac{\textrm{Car's Speed}}{d2}$$
$$\textrm{Car's Speed}=\frac{\textrm{Truck's Speed}\times d2}{d1}$$
So, suppose the lanes are the same size (10 feet wide, say), and the cop is 5 feet off of the highway. Then, $d1=5\textrm{ feet}$, $d2=15\textrm{ feet}$. The speed you need if the truck is going 65 miles per hour is $$\textrm{Car's Speed}=\frac{65 \textrm{mph}\times 15}{5}=195\textrm{ mph}$$
Edit: Some concerns were raised in the comments that this treats the truck as a point. This turns out not to matter. Here's a second picture like the first, but now we have a zone (colored in green) which the truck covers. 
The green triangle gives you a little bit of wiggle room, since you can be covered by the front of the truck or the back or anything in between. However, the total size of your wiggle room does not change while you move (in other words, it doesn't depend on a4). As a result, it should be pretty clear that this doesn't change things much at all - we can think of it as two point-size trucks going at the same speed, and we have to stay between them. Of course, this will give exactly the same answer as the first case - it's really just like hiding behind one point-sized truck.
There is actually one small change, as David notes: If you start out covered by the front of the truck, you can go a little slower than the 195 mph cited above, because you can slowly slide back until covered by the back of the truck. However, if the length of the truck is $L_{tr}=40\textrm{ feet}$ (say), then this change in the velocity is quite small.
For example, suppose that we slide back 40 feet from the front of the truck to the back over the course of a mile. We're going 195 miles per hour, so it takes us 18.5 seconds to go one mile. In those 18.5 seconds, we move 40 feet relative to the truck; this is a speed of about 1.5 mph. So, we can go 1.5 mph slower if we start at the front and go to the back over a mile; taking this into consideration, we get that the speed needed is actually 193.5 mph.
