[RedDevil]

Side barriers are generally bad for MPG as they squeeze the air between your vehicle and themselves. The air you displace has to move somewhere, anything that gets in the way of that is bad.
The exception is with strong side wind; as the barriers block that too that helps.
Most cars have had aerodynamic tweeks of some kind from windtunnel testing. They work well head on, but might not when hit by side wind. My '11 Insight is one example.

I ride in the wake of trucks if I can.
A good truck with low rear shielding on a windless day at 90 km/h (56 mph) can reduce my FC to less than 3 l/100km (better than 75 mpg) on the level.
But don't follow too close, as I found that the turbulence is quite strong close by. That lowers aerodynamic efficiency so it may even increase drag in spite of the relative airspeed being lower.