In the UK, trucks (lorries) are speed limited to 56mph by law, and most of the limiters are actually set to 53mph to ensure they don't break the speed limit (this is based on GPS results). You can drive 2 seconds behind one of these, and if you put your hand slightly out of the window, you will feel a lot less wind when driving this far behind, compared to driving with an open road. Driving closer than 2 seconds gives better results, but is a bit dangerous, so I don't do it. Also, when it is raining, it is too difficult to follow them due to water spray.

I can imagine, if the lorries did 70 (as they did when I visited the US), it would be a lot more dangerous to draft them, and more economical to drive at 50 anyway, but in the UK I get the best of both world (lower speeds + drafting capability)!

I *always* draft trucks in the UK - they go a lot more slowly, never throw anything up, and 2 seconds is a bigger distance than most people leave anyway!.

By the way, I have found that driving alongside trucks/lorries is actually bad,
because the lorry pushes the wind aside, and you get an extra-dense faster moving column of air hitting your car - when a lorry goes to overtake, you can feel a sudden decrease in speed as the wind hits you.