RoyJ wrote:
nickthehunter wrote:RoyJ wrote:
There're many studies showing the closer to the cab a 5th wheel sits (commercial semis), the less drag.
One extreme is euro semis, where the trailer is almost touching the cab fairing. Mercedes got 18 l/100km (13 mpg) at 88k lbs GVW.
The other extreme is stretched show trucks (300+ WB Petes), 5 mpg on a good day.
Best mileage would be a short box pickup with a sliding 5th wheel. Have the 5'er up tight against the cab on the hwy.
The cab of a semi is the same height as the trailer (they use wind deflectors on top of the tractor to ensure this). An RV fifth wheels sits way above the top of the pickup (usually much higher than a TT) and there is still a considerable gap; albeit less then a TT but it is not insignificant. Comparing semis to RV's is Apples and oranges.
It's the differential in height along with the gap that matters. The TT has more gap, the 5er has more height. And the answer to which matters more is anecdotal at best.
I wasn't comparing a semi to a pickup.
I was comparing 2 semis, 1 with larger gap, 1 with smaller. Then 2 pickups, 1 with larger gap (TT), 1 with smaller (5'er).
I do agree typical drag area is larger on a 5'er, but your Cd will be lower. Maybe not Cd*A, which depends on the size of the 5'er.
A short box slider 5'er may have a gap of 3', while a long box TT will have a gap of ~14' (8' box + 1' hitch + 5' a-arm). 3 vs 14 makes a difference.
Good job with the “editing”. 
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