Racing car body modifications for fuel economy

Not many sold in the USA, but there is an active market in Japan- so the stuff is available, but shipping is expensive. L.A. might be an exception.

If you search for the car under it's Japanese name, "Aqua" or its production code "NHP10" you will find some things.

example here. Not an endorsement, I've never heard of these guys before.

Also be aware that this body style was discontinued/updated for 2022, so parts for the original body style will be disappearing soon.

The only mod I know of that legitimately improves fuel economy is dark window tint, because then you don't burn as much fuel to run the air conditioner. It makes a surprisingly big difference. Using a reflective shade on the windshield when you park is also a big help- more than you would expect.

 

They do that for power, not fuel efficiency.

I seem to remember older models having some fuel economy success with warm air intakes. This is the opposite of the racing crowds who want cold air intakes for greater power.

 

Hotter intake temperatures helps fuel to vaporize (phase change from liquid to gas); similar to water at 150°F will boil (vaporize) much faster than water at room temperature. Lower pressure helps vaporize liquids also. Maybe that's why gasoline is pressurized when traveling from tank to injector.
Cold air is more dense and can accommodate more fuel which is why bigger fuel injectors are paired with intake systems (especially forced induction).
Older engines could gain fuel economy from cold air intakes because the car computer (ECU/ECM) wouldn't [necessarily] add more fuel, thus causing the engine to run leaner. I experienced this 1st hand with my first car, a 1998 Corolla that I had put an air intake with an 3" oiled K&N filter. I also used a "sport" muffler, a grounding kit, wrapped exhaust header, and a chip that piggybacked off of the ECU (up to 52 mpg highway; distance/fuel put in).
A major difference between the '98 Corolla I had and a '02 Corolla I sold earlier this year (2022), was that the '98 did not have a mass air flow sensor, just a intake temperature sensor. In other words the '98 "assumed" it was getting air in at the stock velocity when the intake tube I put on it had 75mm (nearly 3") diameter. I was also pretty happy with the louder sound under the hood.

 

A lot of 90s cars used speed density for fuel metering. Some still do; and it's popular with fuel injected race engines.

Metering mass airflow was always more accurate, but it required more expensive parts and more space under the hood. It took a while to fit in.

 

When I owned a 96 Acura Integra, I found out that Integras were raced by enough people (not just a few). They would have fiberglass (fibreglass; FG) body panels made; usually the two front fenders and hood (bonnet). ViS made these in carbonfiber (CF) for the Integra also, the prices were expensive then (2008) as they are today.
A local body shop might know of a FG, FG & kevlar, CF fabricator that can make a hood from a mold of an OE hood (or yours). It maybe cost prohibitive.
The ViS CF hood for the 2010-14 Prius is $1,300 (as of 09/22) plus installation. If Cali has gas for $7.00/G then that's ~186 gallons you could buy instead. How long will it take to save about $1,300 from a lighter weight hood? So say you got +1mpg doing 12,000 miles per year: 12k miles/year @$7/G= $1,680 spent ((12k / 50mpg)x$7) compared to 12k miles /51mpg (235.294 G/year x $7=$1647.05) $32.94 year savings. It take 39 years and six months to recoup that money (1,300 / 32.94=39.465 years or 14,405 days). At the very least a ViS CF hood isn't worth it.
All: Please check my math & assumptions.

 

I'll stop you right there. That would be an extremely impressive improvement for just dropping a few pounds of weight. I think you are making an overly optimistic guess there.

The rest looks good- if you can get a real-world number for efficiency vs. weight you'll be on track to estimate it well.

It wouldn't be a perfect test, but find out the difference in weight between that CF hood and the one you have now. Then put that much extra weight in your car as ballast for a month and see how much your mileage changes. then re-run your math on that.