specifically, they found that the drafting improvement was best at 10ft behind (saving 40% mpg), but even 100ft behind saved 10% mpg.
- 100ft: 35.5mpg, 11% improvement
- 50ft: 38.5mpg, 20%
- 20ft: 40.5mpg, 27%
- 10ft: 44.5mpg, 39%
- 2ft: 41mpg, 29%
more details below
Drafting a Big Rig
Myth: Drafting a big rig saves fuel
They emphasized again and again how dangerous drafting a big rig is: ~3/4 of truck/car accidents are caused by person driving the car and you're driving in the blind spot of the truck. 150ft is minimum recommended following distance at 55mph, so even the 100ft test is considered dangerous.
Small-scale test
NASA let them do a small scale test to study the aerodynamics of a big rig. They captured video of smoke travelling over a minature big rig and verified that there is a low pressure area behind. They then stuck a minature car to a force gauge to study the difference with and without drafting.
- 7 car lengths: 21% drag reduction
- 10ft: 60%
- 6ft: 80%
- 2ft: 93%
Full-scale test
Freightliner lent the MythBusters one of their new
Cascadia big rigs, which they are billing as the most aerodynamic big rig on the market.
Mike Ryan, Hollywood stunt driver, was there to educate Grant on the ins and outs of drafting. Also on-hand was Andrew Smith, test engineer, who helped them hook up a computer to the fuel injection system to accurately measure the fuel consumption.
- 55mph control: 32mpg
- 100ft: 35.5mpg, 11% improvement
- 50ft: 38.5mpg, 20%
- 20ft: 40.5mpg, 27%
- 10ft: 44.5mpg, 39%
- 2ft: 41mpg, 29%
The fuel economy actually dropped at 2ft. Andrew Smith's theory was that at 2ft, Grant got nervous with the throttle as it was difficult to maintain that 2ft gap.
Annotated Mythbusters: Episode 80: Big Rig Myths