Most truckers call it "Bill boarding", but your state trooper will call it tailgating.
In any event you must watch your distance to an 18 wheeler and still reap the rewards of
a few more MPG's. there will be a few truckers that may be annoyed by your presence
too close behind them, but for the most part many of them are oblivious or don't care whats behind them
as they cruise down the freeway.
I would'nt suggest any closer than 3 car lengths. What got me interested in the thought "how good is the MPG's" back there is that my recently purchased 2006 Cobalt came with a scangauge built-in the dash that reads
MPG among other things like trip A miles, trip B miles, the cars mileage, oil condition, etc ... you get the idea.
Anyway I snugged up to one and hit the MPG reset button.....gave it 3 or 4 miles to settle down. And it was reading 50+ MPG's WOW! normally it's about 38 or 39 (sorry not sure what this converts to in Km) Here's another WOW factor for you.....now turn on your A/C!!! you'll most likely loose what ever you gained snugged up to that truck!
To get yourself an aftermarket scangauge for you car google scangauge.....and take your pick. There are even
OBDII (On Board Diagnostic II) chips that plug-in to the port under your dash that are bluetooth enabled ....
then download the app to your phone (they are very specific between andriod or iphone)
hope this was helpful, driumph3D