Avid hyper-milers and NASCAR drivers already know that following closely in the slipstream of another vehicle can reduce the amount of energy it takes to maintain a given speed. Drafting is a known phenomenon but researchers in India wanted quantify how much of an impact it really has. At Jadavpur University they put four cars into a wind tunnel and found that the second car in the series would need twelve percent less fuel to maintain 50 mph and the third and fourth cars needed twenty-one percent less fuel.
They don't say how close the cars were too each other but it was likely a lot tighter than would be prudent on public roads with unknown drivers. The only way this might be practical in the real world is when cars are equipped with vehicle to vehicle and vehicle to infrastructure communications to preemptively warn of potential problems. Even then there are far too many variables such as road conditions, different drivers and other obstacles to make such close following a viable practice.