Think a typical "teardrop" airfoil shape, they are blunt at the front and gently taper off at the rear.

In fact most cars are more aerodynamic in reverse than they are going forwards for exactly this reason - on most cars the front end comes in more gradually (the hood portion comes in first, then enlarges to the passenger section) whereas the rear end is usually much blunter (to maximize storage space).

> most cars are more aerodynamic in reverse than they are going forwards

I tried to find more about it, but I could only find two sources. The first was a Mythbusters episode about a particular Porsche, and the second was a Top Gear episode about a different car. Can you point me to an article that substantiates this?

https://en.wikipedia.org/wiki/Drag_coefficient

If you taper properly before the flat cutoff a squared off back (Honda insight is the quintessential example) is fine because the real turbulence happens a few feet behind the vehicle. At speed a blunt (even if it's rounded) nose just isn't great no matter how you cut it but with careful attention to corners, shape transitions and whatnot you can mitigate that quite a lot (modern crossovers with their bulbous front bodywork make heavy use of this).

This is a subject with a million gotcha's that add up to a lot. Be weary of anyone peddling rule of thumb solutions.

Here's one data point, merely removing the rear spoiler an SUV has the same drag in reverse as it does going forwards: https://airshaper.com/blog/mercedes-eqc-drag-coefficient

If the same car had the fine details massaged for the reverse direction in a similar way to how they had already been done for going forwards (i.e. instead of the rear spoiler, putting a similar spoiler facing the opposite direction at the front of the car), the Cd would be significantly less in reverse.

I'd love to get it running again but cant find decent instructions for swapping the battery system out for a normal alternator and battery.

Bikers do this too BTW if you follow the sport, makes for interesting team type tactics.

There is some thought that automated road convoys of trucks following closely might generate fuel savings this way as well.

Funny, my current car does 24 mpg on the highway at the speeds I like. Half the efficiency and probably mostly due to me.

Here's a study which pretty much confirms my idea of slightly rounding off the nose would be the best thing https://www.transportenvironment.org/wp-content/uploads/2021...

Image here: https://ibb.co/FgZvz6p

The article itself states there are bigger benefits to be had modifying the trailer:

"Further aerodynamic improvements are possible, notably on the trailer. For example aerodynamic add-ons, such as tails and side wings, could help deliver even higher significant fuel savings. Their potential is explored in another T&E briefing. [23]"

The referenced article: https://www.transportenvironment.org/wp-content/uploads/2021...

That article shows a variety of tail modifications which have higher effectiveness than rounding the front.

It also looks like truck in EU are limited to 55mph. When I tow our trailer (just with a normal vehicle), there's minimal difference between fuel economy between 45 and 60. Significant drops come into play at higher speeds.

A lot of long-distance American tractor trailers deploy fold-out cones on their back doors to improve their aerodynamics while driving. They fold up for low speed driving and loading.

Also, fold-out flaps under the trailer for the same reason.

Is it something with state by state safety regulations, like prohibitions on protruding structures that aren't part of the main body? Or differently-defined overall length restrictions? Does a given long haul truck have them deployed in the middle of the country, but then they're removed before getting to the coastal states?

There's an interesting mis-alignment of incentives. It comes down to who owns the truck vs the trailer.

* Truck owner pays for fuel.

* Trailer owner pays for the flaps.

So, unless you own both the truck and trailer, there's little incentive for trailer owners to install them.

In vehicles, there's a lot of additional, interesting factors at play - like engine efficiency.

https://en.vda.de/dam/vda/publications/FAT-SCHRIFTENREIHE%20...

And the front aerodynamics dépend more on the details than it's length. Some aerodynamic designs have shockingly steep fronts.

https://en.m.wikipedia.org/wiki/Schl%C3%B6rwagen

The most effective aerodynamic changes are on the back though.