In a bid to reduce wind resistance and drag, the tractor has been redesigned to be significantly more aerodynamic plus the trailer gains side skirts; "wide based" and/or high-efficiency tires, whose reduced rolling resistance improves fuel economy; and the replacement of one regular rear axle with a tag (load-bearing only) axle, which reduces the truck's weight.
I really wonder why it has taken so long to re-visit the aerodynamics of freight trucks - crude though it may be, the photo above is from research Nasa was doing into air flow testing on aerodynamic trucks in 1975. That's 32 years ago and yet really nothing has changed with the trucks you're likely to see on the roads today.
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