I was a semi-truck driver for a while. A lot of "fuel savings" gimmicks amount to sticking a card on your bicycle frame so the spokes hit it a lot and make it sound like you're going fast. It's bullshit. Why do some companies do it? Tax breaks and grants. Do they work. No. Why don't all the companies do it? There's actually a limited amount of companies who can apply for the breaks because the laws are incredibly fucking stupid when it comes to commercial trucks (and to drivers). Similar how there are limited amount of carbon tax credits you can apply for. Some companies just don't waste their time. I have nothing against protecting the environment. But bullshit gimmicks won't do a thing.

From looking at it on Google images, could it be that this device has other downsides? Making doors more awkward to open, increasing turning radius etc?

A significant percentage of semis you see are nowhere near fully loaded.

Especially for local deliveries- the truck is often half-empty.

Don’t you mean half-full?

When you're doing deliveries half-empty is the optimistic take.

Can’t leave me hanging, brofus

Don't you mean twice as big as it needs to be?

The marginal cost of dragging 40ft of air is so small compared to the hassle of trying to determine which truck is the correct one for today’s deliveries and pickups, so the companies tend toward having all trucks the same size.

Or... semi full. Ba dum dum tsss

That's odd. Airlines would kill to install a cheap device on their fleet and save 5% on fuel. If diesel is $3/gal and the semi holds 200gal, that's about $30 savings per fillup and would pay for itself quickly.

The fuel tanks on an aircraft are a major portion of the weight of the whole craft. So make engines more efficient, and you can take less fuel, which means the plane is lighter, so you use even less fuel and can take even less, etc. As you reduce the amount of fuel needed to go a certain distance you can go further and convert two-leg flights to single hops, which saves all sorts of other costs.

The fuel tank of a semi truck is not an important proportion of the weight of the whole vehicle, so it does matter, but not so much.

Wind resistance goes up by the cube of the velocity, it would be a much larger savings on an airplane.

Still even at interstate speeds a 5% savings pays for itself pretty quickly.

They seem to be reasonably popular around here. I see them used on trucks on the Interstate probably more than 50% of the time.

They may not be worth it for short haul trucks, but long haul trucks I bet the savings adds up quick.