Beauty is in the eye of the beholder
Trailer gap was never even a consideration back in the 90’s. As a matter of fact, it was another 10 years beyond that decade when things began to change. Back in the day, you could NOT talk a customer into ground dump exhaust… everything was independent, with pipes vertical behind the cab. This was “The Look”.... What changed?
Price of fuel, EPA standards, Model restrictions. Well, these are somewhat related as OEs funneled development efforts to align with the first two. You see, vertical exhaust stacks add at least 20 inches to the trailer gap. This is what is required to clear trailer swing. Trailer gap is the measurement from the back of the cab or sleeper to the forward face of the trailer.
If you have side extenders on the chassis, or if you have a reefer unit on the trailer, these items are omitted from the measurement. Studies have shown that the more you can close this gap; the less resistance will be encountered as air flow moves over the chassis and trailer as a unit. Larger gaps permit air to flow between the chassis and trailer, creating a vortex resulting in drag. This area accounts for 25% of the total combination drag. And the best measurement for minimizing this loss is 42-inches maximum.
What you should know is how the dimension, and associated lack of efficiency, translates to operational costs. Lowering the gap by 10 inches provides a 0.5-1% increase in fuel efficiency. So, if we apply that to the independent exhaust that was popular 15-20 years ago, we see a loss of 1-2% fuel efficiency.
Today the industry is ALL about efficiencies. The past two Greenhouse Gas (GHG) emission levels have focused on this. As chassis styling changes, and powertrain evolves, it makes sense that OE product offerings align with vision of efficiency. We are all in this together. And the results are impressive, fuel economy is at an all time high, which lowers operation cost and provides increased revenue for customers.
Of course, you see many new trucks with exaggerated wheelbases, “The Look” for certain applications. It is doable. You can, on some models, still get vertical tailpipes behind the sleeper. And truly, if your customer wants the vehicle stretched out, fuel efficiency is less important, and the vertical stacks complete the look. In these cases, the benefits of a 42-inch gap is not worth the debate.