BTW, I noticed that there may be up to a 3MPG gain by drafting cars at 75~80MPH with modern aerodynamics.

I noticed the effect behind a mazda, shifting back and forth lane on a steady slight uphill (I-5) this week-end. i think it was a Kamm design.
https://tbn0.google.com/images?q=tbn:...4415-img_0.jpg

I also noticed that behind some other cars/trucks and even semis I did not always get a gain, and sometimes a loss.

However this has to be verified more. Though 3MPG gain is significan, the scangauge often varies wildly, and the measure is hard to validate as the road goes up/down, cars change speed and I need to manage the cruise control.

Use the trip computer part of the SG ;) Set it to current then set the display to mpg. When you're ready to take a reading - hit reset. That will average your FE and smooth out the FE variation. Just be sure to peek at the gauge just before your done ;)

Just remember that drafting can be very dangerous. The biggest danger is the person you're drafting suddenly changing lanes at the last moment to avoid an obstacle or other slow moving car.

In the case of a semi, there's still a benefit to be had at a safe distance (just not as great) ;)

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Here's a good read on fairing design for HPV's - but concept for general fairings is the same -- try to minimize the variation in pressure across the surface. https://www.recumbents.com/wisil/barr...ringdesign.htm

Given enough length, laminar flow will naturally go transitional and then turbulant... 3D shapes are pretty much impossible to calculate by hand, but if you look at a flat plate (for example) with airspeed at 55mph - the flow goes turbulant at around 2.3 feet from the leading edge (using Reynolds number = 1x10^6 criteria). But turbulence is a funny thing, where laminar ends and turbulence begins is partially arbitrary. But a gently curved shape will move that transition point back... Flow control also helps (think golf ball dimples, tennis ball fuzz, shark skin, etc.). Oh man, I should have picked aerospace :p

^My point is - the biggest reason for diminishing returns is nature :p

Somebody once noted really good results drafting a bus due to the bus body reaching very low down. Some box trucks also reach much lower than typical semis.

And again, don't draft close. Just not worth it if you get smacked up.