Featured Gen 6 Prius engine will be a “game changer,” achieve a 53% thermal efficiency

BYD had a 43% efficient engine back in 2021. The next gen being better isn't a surprise.
Development of 43% Brake Thermal Efficiency Gasoline Engine for BYD DM-i Plug-in Hybrid

The not using traditional rounding is seems to be because of one up manship with Geely. They announced a 43.32% efficient engine soon after BYD released the 43% one. There is a government org certifying these values in China. BYD has a 46.5% thermally efficient engine; the 46.06% is the production model.
Geely and BYD argue over engines, plus other fallout from PHEV tech announcement

You were the one claiming it will be 50% here.

Currently, it is in a new Seal. BYD Seal 06 - Wikipedia
I'm sure BYD will use it elsewhere.

The Metro barely beats the Prius in the combined rating. If the EPA didn't have a thing for whole numbers, the difference could be as low as 1/10 or 1/100 of a mpg.

Cd is only part of the aero equation; the other is frontal area. The Metro is two size classes smaller than the Prius; it is a half foot shorter(height) and thinner. Wind may push harder on it, put there is good amount for it to push against. Then the Metro isn't saddled with ten more years of emission regulations. Fueleconomy.gov doesn't post emission ratings for cars that old, but it probably wouldn't pass the minimum of 2004, let alone the clean rating the Prius achieved.

It also had a 0 to 60 mph time of 15 seconds. With the patience required to drive that Metro regularly, a driver in the Prius could likely reach the original mpg rating

In that light, the two being close isn't that surprising. The Metro engine is working near its thermal efficiency peak most of the time. Which leaves it with little reserve power, and low power performance. The basic hybrid concept is to use such smaller engine, and have the electric side make up for the performance limitations doing so entails.

 

Yes, yes and yes.

But that wasn't the point.

A Geo Metro has around CdA of 6.24 (the 2-door Xfi may be different) whereas the 2nd gen Prius is about 7% better at 5.83 yet the Metro Xfi also has a new highway rating of 52 mpg whereas the 2nd gen Prius is only 45 mpg, 15% lower. How does a car with 7% better total aerodynamics get 15% worse fuel mileage?

Yes, emissions regulations and such has a lot to do with it, but efficiency is efficiency. Excuses, reasons or not, one is more efficient than the other. And if we compare that efficiency with, say, a diesel freight truck, it may be even more efficient. It may be apples to oranges, but if we get a +50% efficient engine in a 3-row SUV with a CdA of 28 and that costs $98,000, that efficiency is no longer all that impressive.

 

At first glance, looks like the Prius has about a 24% larger cross section, and is about 80% heavier. And in 1994, the National 55 MPH speed had not yet been fully repealed.

 

• The overall drag on the Prius is still less, despite being bigger.
  
• Weight isn't as much of a factor as one would think, especially for highway fuel mileage ratings, but maybe you have a point here to a degree.
  
• The 52 highway MPG the the current revised fuel mileage in terms of 2024 EPA fuel mileage testing standards, which has nothing to do with 55 mph highways. The original highway fuel mileage rating in 1994 in the Geo Metro Xfi when there was a National 55 mph highway law was 58 mpg.

 

Weight and smaller tires do affect highway fuel economy, aero isn’t the only consideration.