On The Fence - Help me!

This web page shows graphicly or a table of the maximum speeds:
Map of maximum posted speed limits on rural interstates

As a general rule, posted 70 mph traffic is typically enforced only when speeds are within 10% or less due to errors from aftermarket tires. For example, in my NHW11 tire testing I have a 6% error. My GPS measured 75 mph shows 71 mph on the dash. But speed limits also depend upon local enforcement and some jurisdictions are notorious for having a lower ($$) tolerance for exceeding the speed limit. As a general rule, follow the speeds of locally tagged cars. Speeding out of state cars are 'revenue enhancers.'

Bob Wilson

 

Hey Tek...just wanted to say I bought the same car you did about 3 months ago, and it has slowed me down considerably! BUT, I don't mind, as it is a conscious slow down...it feels so good to be saving gas, saving money, and saving the environment. Congrats and Enjoy!

 

This is one car, where "slowing down" yields immediately more $$$ in your pockets.

 

My data suggests an optimum acceleration power is just a small 1/4 dark, part of the PWR tit on the HSI display. This corresponds to maximum EGR flow and the knee in the exhaust temperature. This is roughly 4,100 rpm. Any lower and acceleration is slower. Any faster and more energy is lost out the exhaust pipe.

Another approach I just learned about deals with starting from a standing start say at a light or stop sign. Hit "EV" and let the car use electric power to get rolling to EV max speed. Thereafter, just use the 1/4 dark on the PWR tit. Once you reach target speed, hit "EV" and/or cruise control and enjoy the ride.

I've also started experiments on using a downgrade to handle the vehicle warm-up. No data to report yet.

Bob Wilson

 

B is arguably better for MPG, but guaranteed better for your safety from those behind you who are looking to merge into highway traffic as well

 

Bob, while you certainly do a much better job of collecting data before you form an opinion than the average PCer, you don't seem to get terribly spectacular (just ever so slightly better-than average) mileage, judging by your signature. Any reconciliation here?

 

Uh, is there a problem with being "above average"? <GRINS>

Actually, I have 'pegged' the MPG indicator for non-trivial distances, over 10 miles. So what distance qualifies as 'acceptable'?

I learned a long time ago that no matter what mileage is achieved, there will always be folks who think some arbitrary distance is required. Understand I have no problem with those who feel compelled to achieve some long distance threshold . . . but since you bring it up, what is that threshold?

When I was active in GreenHybrid.com, I looked at the various NHW11 mileage records. There was one set of 'one time wonders' who posted remarkable mileage numbers and made sure everyone know about these 'one hit wonders' . . . and disappeared. But I also noticed there was a wide spread of mileage records that were being kept up, over five entries and current:

• low: 39 MPG
• high: 56 MPG

So I asked both the highest and lowest to understand what was going on.

It turns out the lowest mileage number came from a Boston intern, doing his residency. He lived at the bottom of a hill and on the other side, at the bottom, was the hospital he was doing his residency. His mileage was completely understandable and I would never accuse him of being a slacker. Because as a resident, he was doing 60+ hour weeks and he really didn't have time to fiddle with trying to maximize Prius mileage.

So I don't begrudge others their mileage. After all, our Prius are a means to an end, a better life. If some get below average, I don't judge them. If some get higher, I'm glad but also realize it is often because of accident, the fortunate location of home and and work.

So instead, I conduct well controlled experiments to find out what variables impact mileage. I do A-B-A tests and share the results. I have my signature and sure it shows "above average" but think about 'Lake Woebegone' where

"all the women are strong, all the men are good looking, and all the children are above average,"
​

The irony is this evening I measure 195/70R14, Sumitomo T4s on the front wheels at 40 mph and found they delivered 74.2 MPG at 82F with our 2003, NHW11 Prius. Have you made a similar contribution to our Prius knowledge base? Perhaps I missed the posting.

Look, I really don't care whether or not you do any sort of controlled testing. I really don't care what mileage you achieve on a single trip or for that matter, anyone else. I am really interested in reproducible results, ones anyone reading my postings and seeing my data can have confidence that they can achieve the same or at least understand why not.

Now the simple answer is drag, both rolling and aerodynamic, sets the energy needed by our vehicles. Furthermore, the ICE, transaxle, wheel bearing and tires dictate the tank-to-wheel efficiency. We can play 'games' to optimize both but does it really buy us or anyone else any good?

I do plan to improve my NWH11 efficiency AFTER I take care of NHW11 longevity. No fuel savings can make up for having to dispose of the car early. But as the longevity issues are addressed:

• reduce drag - the most cost-effective enhancemept
• topping cycle - getting some good from the waste heat
• transaxle and running gear - using all of the energy
• ICE efficiency - the least productive area of investigation

Bob Wilson

 

I always wondered what the official technical term for that was. Thanks for enlightening me.
Cheers
~T

 

sorry

 

i's so glad you asked!

 

Good answer.

And just so I'm clear, since I'm not sure the original message was - I wasn't trying to take a dig at you with my comment. I just respect your opinions more than the average comment, since you put some thought and trials into them. There are many other opinions here that I just plain ignore because I see little sense (and even less evidence) behind them. And for the record, I get about the same mileage as you do, although I've only had about three tanks so far. I'd like to think that I'd be doing better if I lived in an area with less traffic, fewer enticing freeways, and a longer commute to work (though it hardly ever gets used for that - a bicycle is my first choice vehicle).

As far as my own experience, I've tried various strategies suggested here, and found that I get the best results when I stay out of the PWR bar. No real thorough testing, which is why I don't generally share that opinion - I just saw my mileage dropping like a rock when I tried it for a while, and switched back to my original style. I imagine that it will take me quite a while to get a good improvement at my mpgs.

A better question to ask you would perhaps be - have you measured fuel economy when accelerating with none of the PWR bar lit and half of it lit? What were your results from this? (I fully expect you have, and it's probably already on PC somewhere. Probably several times. ) And I don't think I mentioned any particular distance threshold (other than, implicitly, your total distance driven), but in general, I'd say testing should be done such that battery SoC effects are minimized; a tank seems like a convenient threshold, if the SoC is completely untracked. Tracking by percentage might be needed if tests only last a few miles, probably. And even that isn't a terribly good method, since I expect there could be an efficiency difference between, for example, charging the battery during regen and charging directly from the engine (for example, perhaps the latter could be used by the computer to hit a particularly efficient operating point, while the former would involve some extra mechanical losses).

I've tried a few of the other suggestions I've seen, like trying to get ~20 kW output (according to the ScanGauge) from the engine for peak efficiency, and that seemed to drop my fuel economy as well. I haven't done any real controlled testing - and as far as I'm concerned, it's pretty hard to get truly controlled testing, short of building a robot and dropping the car on a dyno. But, perhaps my lower-than-expected economy from those experiments may be due to local factors, rather than this not hitting the peak engine efficiency. With a large number of other vehicles, traffic lights, and stop signs, perhaps accelerating more slowly is better in my case - it may be that if I accelerated at the most efficient rate, I would be at full speed by the time I saw the next obstacle, while slower acceleration means that I spend more time at lower speeds, and may stop accelerating when I spot an obstacle. Just an untested theory, though - and likely to remain mostly untested by me, since I lack the commitment and interest (despite being an engineer... ), and since I drive so little - about 2000 miles so far, about 600 of which were in a single weekend, and several other 200-ish mile trips on other weekends.