Absolute maximum MPG with a 2nd gen Prius + aero ?

[freebeard]

Nice work, but the original is not my favorite. The cuts they use result in a flat top and lots of seams.


hubpages.com/autos/Top-10-Cars-That-Make-Great-Hot-Rods

I can't remember the name of this car, but it is a chop done right. The roof is cut off and the body is quartered, narrowed and shortened. The curvature of the top is preserved and lots of the welds are interior to the body.

[Gasoline Fumes]

Originally Posted by Cd

According to A.I., installing a cheap cat will last maybe 6 months. Sometimes less.
Many times, it will cause codes to show up.

California doesn't even allow it, since aftermarket cats have about one forth the amount of metals that are needed to clean up the emissions.

There are only two aftermarket cats available that are approved in Cali, and well ... they are NOT available, because they have been on backorder forever. They are also pretty expensive too. $ 475 and up.

Cheapo cats can be had for under $170.

OEM cats are $2,500 and that is more than the worth of the car !

It seems to me that removing the cat would be like removing a turbo off a race car because you don't want it stolen, so you replace it with a cheap junk one.

The cheapo cats should last a few years, not months. But you're right, you can't do that in Cali. Or NY (if anyone actually checked). I still have the original cats, but a friend has successfully used the cheapest aftermarket ones he could find. If you can keep the original cats from being stolen, that's definitely the best option.

[Cd]

Originally Posted by Gasoline Fumes

The cheapo cats should last a few years, not months. But you're right, you can't do that in Cali. Or NY (if anyone actually checked). I still have the original cats, but a friend has successfully used the cheapest aftermarket ones he could find. If you can keep the original cats from being stolen, that's definitely the best option.

Well then, I'll just try and protect what I have and try not to worry too much. The car already has over 170k miles on it , so the cat(s?) are likely clogged up already and not filtering most of the pollutants.
Come to think of it, making it super hard for a thief to remove the cat might actually cost more. The theif would likely slit all my tires and bust out my windows just for spite.
@Freebeard : I agree about the roof chop. I too prefer the curve top roofline.

Hey, so back to my question about the test, do either of you guys see a link to the drone video, or a breakdown of the test results for the aeromods at a steady 65 mph ?

[Xist]

When I tried to find a Prius, it seemed like the cheapest way to replace the cat was to buy a broken down Prius which still had the CC, which seemed like the first part sellers sold.

[Cd]

I asked A.I. about the Prius again : "So no way to get 65 @ 65 in a G2 Prius. Even if rolling down a mountain, the ECU is going to keep the engine at high RPM, right ?"

Not quite — let me untangle that, because it’s partly about the limits of the Gen-2’s power-split transaxle design.

1. Steady 65 mph on level ground

A stock Gen-2 Prius will not reach 65 mpg at 65 mph, even with perfect conditions.

Best case you’re in the low-50s (stock) or mid-50s (with aggressive aero mods).

The hybrid system and Atkinson cycle engine are already tuned close to their sweet spot, so the physics ceiling just isn’t high enough.


2. Rolling downhill at 65 mph

The ECU won’t force high RPM just because you’re coasting downhill. What happens depends on state of charge (SOC) of the traction battery:

If the battery has room, the car will use regenerative braking, and the engine will usually stay off (RPM = 0).

If the battery is full (often after a long descent), the ECU will spin the engine (around 3,000–4,500 rpm) as a “compression brake” to dump excess energy, even though no fuel is injected. That’s the high-RPM “why is my engine revving on a downhill?” behavior Gen-2 owners report.



3. Why you can’t game it into 65 @ 65

Even on a downhill, if the engine is being spun for braking, it’s fuel-off, so your mpg readout goes infinite — but that’s not sustainable flat-ground performance.

On flat roads, the Gen-2’s combo of CdA (~0.58 m²), weight, and drivetrain efficiency just don’t allow 65 mpg at 65 mph. Toyota didn’t reach that until Gen-4/Gen-5.



---

So:

Downhill: yes, the ECU can spin the engine, but it’s not burning fuel — just protecting the battery.

Flat 65 mph cruise: no realistic way to see 65 mpg, even with aero mods. Mid-50s mpg is about the ceiling.


👉 If your target really is “65 mpg at 65 mph, flat road”, that’s in Gen-1 Insight / lean-burn VX territory, not a Gen-2 Prius.

Do you want me to run the numbers for a Gen-1 Insight at 65 mph, so you can see why it does hit 65–70 mpg where the Prius can’t? - endquote

Well that throws that Idea in the dumpster. 😡
I'm glad I didn't waste my time aeomodding the car.
I had hope that I'd be pleasantly surprised with Darin's boattail testing at 65 mph.
But since the data was apparently never posted, I guessed at his quote about all the aeromods together adding 17.5% at 65 mph.
Looking at what a stock 2004 Prius does at 65 mph and adding the 17.5% amounts to around 52 mpg.
His earlier tests of getting 73 mpg at 50 mph confirm this.

[Cd]

​Answers from yet another A.I. ( Sometimes they don't agree on things ) : " No, it is highly unlikely a Gen 2 Prius (2008) could achieve 65 mpg at a steady 65 mph, even with extreme aerodynamic modifications. While a low drag coefficient is crucial for high-speed efficiency, the car's engine sweet spot and the physics of the drivetrain are the limiting factors.
​The Limiting Factors
​The key issue isn't just wind resistance; it's how the Prius's hybrid system operates.
​Engine Sweet Spot: The Gen 2 Prius is designed to operate its 1.5L engine most efficiently at a relatively low and narrow RPM band, typically around 1,700-2,300 RPM. This is the "sweet spot" where the engine produces the most power for the least amount of fuel. At highway speeds like 65 mph, the car's gearing and drivetrain force the engine to run at a higher, less efficient RPM. This is in contrast to the "pulse and glide" hypermiling technique, where the car is accelerated to a target speed (often 40-55 mph) with the engine, and then the engine is shut off to glide on battery power or momentum, but this is impossible to do at a constant 65 mph.
​Aerodynamic Drag: While aeromodding a Gen 2 Prius from its stock 0.26 cD to below 0.20 cD would significantly reduce wind resistance, the power required to overcome drag still increases with the cube of speed. Even with a theoretical 0.15 cD, the horsepower needed to maintain 65 mph is substantially more than at lower speeds. This higher power demand requires the engine to run constantly and often outside of its most efficient range, which is the exact opposite of what the hybrid system is designed to do.
​The Hypermiler's Reality
​Many experienced hypermilers—drivers who use specific techniques to maximize fuel economy—on the CleanMPG forums and elsewhere have documented their results with Gen 2 Prii. They have achieved impressive mileage figures, sometimes in the 60s and even 70s mpg, but this is typically done at lower average speeds and by employing techniques like "pulse and glide" or drafting. These techniques are not sustainable at a constant 65 mph on a typical highway. While some have reported achieving high 50s or even low 60s MPG at 60-65 mph, these are often "perfect storm" scenarios with flat terrain, no wind, and careful use of cruise control or an external throttle controller to keep the engine in its sweet spot.
​In conclusion, while extreme aeromodding would certainly improve the fuel economy of a Gen 2 Prius at 65 mph, it wouldn't be enough to counteract the fundamental inefficiency of the engine at that speed. The car's hybrid system, designed for urban and suburban driving, is simply not optimized for high-speed highway cruising. "

What a huge bummer.

[Logic]

Originally Posted by Cd

​
​In conclusion, while extreme aeromodding would certainly improve the fuel economy of a Gen 2 Prius at 65 mph, it wouldn't be enough to counteract the fundamental inefficiency of the engine at that speed. The car's hybrid system, designed for urban and suburban driving, is simply not optimized for high-speed highway cruising. "

What a huge bummer.

What about adding extreme engine modding?
This one runs a 30 to 1 AF ratio and 30 to 1 compression to give 50 mpg on a Talon.
https://ecomodder.com/forum/showthre...awd-28776.html

[CigaR007]

I am personally aiming for 60 MPG at 60 MPH with the CR-Z.

I would be happy with that. Been trying to work on my extended kamm-tail for the past year, but a lack of space (no workshop or garage suited for it) has made it difficult. My best tank has been 54 MPG, but that was under ideal conditions. My Summer averages are usually 50 MPG, mostly highway commuting.

Pretty sure my kamm-tail with rear wheel covers will allow to me to reach 60MPG, but that takes time and effort to implement properly.

I encourage you to tackle the challenge nonetheless. Pick the right platform and work on it gradually.

[freebeard]

60/60 sounds more doable than 65/65.

It looks like the active grille block concluded in April. Do you have work-in-progress pix or drawings? Or a list of materials and methods?

[Cd]

Originally Posted by freebeard

It looks like the active grille block concluded in April. Do you have work-in-progress pix or drawings? Or a list of materials and methods?

Eh ? I don't understand what you are asking.