[0]: https://expandedramblings.com/index.php/fedex-statistics/
And UPS has placed an order for 125 of them (https://electrek.co/2018/03/26/tesla-semi-electric-truck-ord...) as well.
You're right that this isn't going to radically transform the country's freight fleet overnight, but I think it's a signal that these companies are willing to switch to EV technology if it works. The next hurdle now seems to be Tesla's ability to execute.
Do we know what fraction of FedEx’s annual truck purchasing this order represents? Fraction of flow is a leading indicator fraction of stock (which you helpfully provided) [1].
Mazda (of which Toyota has a fair stake) is rolling out their second generation SkyActiv which uses spark controlled compression ignition (SPCCI), in conjunction with a lean supercharger, to switch between spark ignition and compression ignition based on engine load. They've not thrown out a thermal efficiency number, but it's likely in the ballpark of 43-45%. They're targeting 56% thermal efficiency for the third generation of the tech, which is already well into its development, and we'll see sometime after 2021.
The current benchmark is Mercedes' Formula 1 engines, which are 50% thermally efficient, but understandably cost an arm and a leg.
This is excluding other things like Koenigsegg's FreeValve which uses pneumatic-hydraulic-electronic actuators (PHEA) to replace the camshaft. The only licensee so far is the Chinese manufacturer Qoros, which is a shame as Koenigsegg got pretty significant adoption of their FlexFuel tech by GM, Ford and Chrysler in the US (> 17 million vehicles sold).
(I am comparing just the weights of the power plants, not the power plant + drivetrain which would include gearboxes, differentials and electric motors).