I'll try a link to SAE International technical paper 2013-01-2414, by Hirz & Stadler, Graz University of Technology.
It's semi-trailer technology.
The long and the short of it was:
* they tapered the trailer roof only and tested.
* then they tested this configuration, plus adding plan-taper boat-tailing.
* with the roof-only modification, wake area was reduced by 13.7%, which reduced drag by delta-Cd 15%, increasing Diesel mpg by 6.5%, at a presumed 83-km/h.
* with roof taper and side taper, the wake was reduced by 30.95%, which reduced drag by delta-Cd 23%, for a Diesel mpg improvement of 10.2%.
* without the plan-taper boat-tailing, there's a 34.7% drag penalty.
https://www.sae.org/publications/tec...-2414/preview/
* Averaging the two, it works out at about 4.38% mpg increase/ 10% drag reduction ( Diesel engine @ 83-km/h )
* the Graz model is :
-648-inches ( 54-feet ) in length.
- 100-inches ( 8.333-feet ) in width
- 156-inches ( 13.0-feet ) overall height
- approx. 10-inches ground clearance
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1) the 'bent' roof exposes the flow to deceleration
2) the deceleration increases static pressure
3) static pressure behind the rear panel increases ( base pressure )
4) increased base pressure reduces pressure drag
5) reduced pressure drag reduces total drag ( the reason for 'aerodynamics' )
6) adding plan-view tapering amplifies the effect for maximum drag reduction, highest fuel economy, range.
7) gasoline has only 81.76% of the chemical energy of Diesel, so gains for a gasoline-powered rig would have to be adjusted down accordingly, plus a loss in thermal efficiency in ICE gasoline vs ICE Diesel.