Sharp MPG drop at higher speed

[cyclopathic]

Originally Posted by cRiPpLe_rOoStEr

A simple rear spoiler might already lead to a good improvement in highway mileage.

It already has one.

[Hersbird]

I would say Toyota optimized it for the EPA test, real world be dammed. The EPA test doesn't cruise at freeway speeds. I really wish they had a EPA test that showed what a car used on flat ground, standard atmosphere pressure and temp, at a steady 70 mph. I guess it hard to believe some people actually do just get on the open freeways of America and drive for hours until they have to pee.

[mort]

Originally Posted by cyclopathic

Car: Prius C.
...
was traveling last weekend. After stopping for gas and pulling back to interstate, "from the start" MPG was holding on at steady 38-39MPG. This is on flat surface with cruise set at 73mph,
...

It normally gets 58-60MPG in commute.
...
According to calculator above the load should have gone up by 25%, and if you consider torque to be constant RPM (load) had gone up by ~50%.

Hi cyclopathic,
What's the problem? If you take a load (power) increase of 25% and a speed increase of about 20% You get a work increase of 50% which should be the same as the rate of increase of fuel consumption, if the engine efficiency is unchanged.
-mort

[cyclopathic]

Originally Posted by mort

Hi cyclopathic,
What's the problem? If you take a load (power) increase of 25% and a speed increase of about 20% You get a work increase of 50% which should be the same as the rate of increase of fuel consumption, if the engine efficiency is unchanged.
-mort

I punched the numbers into recommended calculator 2800lbs, ,28 Cd, .31 engine, .92 transmission efficiency, etc and calculator shows 53 at 65 and 42 at 75. Test snows 53 at 65 and 38 at 75 consumption. Calculated drop is 21% and actual is 28%, 1/3 worse.

[digital rules]

Clean MPG'S steady state MPG test on the Prius C-3. Pretty cool stuff.


CleanMPG Forums - View Single Post - Steady State Speed vs Fuel Economy results

[serialk11r]

Uhh...the Prius C uses the 1NZ-FXE, exactly the same powertrain as the 2nd gen Prius but with worse aerodynamics. If I am not mistaken about my Prius generations, the 3rd gen uses a 2ZR-FXE which is a bit more efficient.

The 2ZR has lower valvetrain friction than the 1NZ, dual VVT-i, and they were probably more aggressive with the cooled EGR flow. It's only ~4% more efficient though.

The only reasonable explanation is that your car was simply consuming more power. Maybe there was a bit of a headwind. As you can see from the BSFC chart that was posted, the mid 2000rpm range is where the efficiency peak occurs.

[redpoint5]

I don't own a C, but the MPG sounds in the ballpark for the speed traveled considering we don't have enough info on the other variables (elevation gain, wind speed, temperature...). My gen III probably averages 45 MPG at that speed.

[cyclopathic]

Originally Posted by redpoint5

I don't own a C, but the MPG sounds in the ballpark for the speed traveled considering we don't have enough info on the other variables (elevation gain, wind speed, temperature...). My gen III probably averages 45 MPG at that speed.

My Gen3 had done ~48 there but this is with Energy Savers.

There is no elevation gain on that road, and trees along to block most of the wind if there were any.

Issue isn't that C is getting bad mileage, it is worse than it should be when explained by aerodrag alone.

[Fat Charlie]

Originally Posted by cyclopathic

If you mean it is changing ratio then it is.

The fact that a CVT does the thinking for you doesn't change what it's doing.

Changing ratio is... shifting.
Changing to a lower ratio is... downshifting.

People get so wound up about scary things like clutch pedals that they forget what's actually happening.

[aerohead]

Originally Posted by cyclopathic

I punched the numbers into recommended calculator 2800lbs, ,28 Cd, .31 engine, .92 transmission efficiency, etc and calculator shows 53 at 65 and 42 at 75. Test snows 53 at 65 and 38 at 75 consumption. Calculated drop is 21% and actual is 28%, 1/3 worse.

There will be a normal seasonal variability to mpg with any vehicle.
There will be 'cold-start' mpg variability vs 'hot-start'.
Steady-speed mpg is evaluated with a fully-warmed vehicle,after 30-miles at 50-mph.
Journalists who've done long-term tests with the Prius C report up to 58-mpg.