You are correct, gpsman1.

However, total efficiency DOES matter because trip times over the same route will vary.

Example; I commute into center city Philadelphia every day. The route and the distance (36 miles) never change, but the elapsed time DOES change; sometimes by a LOT.

I can do the drive in 50 minutes, or the drive can take 4 hours (if there's a wrecked tractor trailer or snow storm).
In this scenario, with a conventional car, the engine is burning fuel continuously for either 50 minutes or 4 hours. With a hybrid this is not the case.

So, it matters.

 

What matters?
What is it you want to know? Ask, and myself or others may be able to tell you.

In bumper to bumper traffic where you are crawling, the engine in a hybrid will run about 1/3 of the time... ~20 minutes per hour ( I've measured 14 minutes per hour in mine when the weather was ideal ).

 

A lot of gas stations charge for more gas than they actually deliver. If the pump says it put in 10 gallons when it actually put in 9, it is the same as if the station owner got a higher per-gallon price for the actual 9 gallons. The reasons for the "error" include temperature compensation devices not being used or used incorrectly, allowable margin of error in calibrating the pumps, and plain old fashioned fraud. In most states, the state sends an inspector out once a year to check the fuel pumps by pumping fuel into a can of known volume, with calibration for temperature and pressure. But, in most states, the stations know exactly when that guy is going to show up. (There are no "surprise" inspections.) So, the day before the state inspector comes, they calibrate the pumps. Then, the day after he leaves, they calibrate them again.

Frankly, I trust the fuel metering devices in the vehicles a lot more than I trust the gas pumps. The vehicle manufacturer has no incentive to cheat. The gas pump owners have huge incentives to cheat, along with plenty of opportunity. A few percent of "error" in the gas pump can easily double their profit.

 

Steve- that is rare, perhaps 1 in a million stations that would intentionally do that. And anyone, you or I could take a 1 gallon calibrated container to any station at any time (without notice) and call them on the carpet.*

Are there un-intentional errors... probabaly... and the errors are probably very small, in reality. Could we get "cheated" 1%... sure. More than that... doubtful.

*Now that you mentioned it, I'm going to try that sometime.

 

where to begin? . . .

1. Regarding odometer accuracy: car magazines routinely note speedometer accuracy in their test results. (They base it on triangulation of numerous measures, including purpose-built aux test gear, known measured distances on race track, and GPS.) It's been years since I read a test with more than a 2 miles per hour error at 60 miles per hour. Usually a 1 miles per hour or 0 miles per hour error. I don't recall seeing a trend in whether that's 1 miles per hour high or low. Anyway, routinely, a <2% error. I haven't seen that bit of empirical evidence come up in any posts on this issue yet.

2. Really, there shouldn't be too much variance car-to-car within a model. The ratios for everything except tire diameter are fixed. That means the only variables are 1) model of tire, 2) inflation pressure (a function of temperature in here too), and 3) (maybe) road surface. Shouldn't be more than a 1% variance anywhere. What's the evidence for the claim of variance car-to-car?

3. Mile markers *are* done in 3 dimensions, because they're done with vehicle-mounted measuring wheels. I know that often they're a bit inaccurate marker-to-marker, and sometimes wildly inaccurate, and I don't know why (government workers? I am one, so I can say that), but they *do* account for the Z axis, and in most cases when averaged over significant distance they're quite accurate.

4. When this question first came up in another thread, I suspected maybe we had figured out the reason for the variance between computer and hand mileage calculations in the HCH II (as others have noted, the display is usually 1.5-2 miles per gallon lower than hand calc). Measuring by time against mile-markers in a couple different trusted straight highway stretches (GPS verified), mine comes out no more than three-tenths of a second off at 75 miles per hour over multiple runs. (That's a max difference of something like .8%.) This was done with Blizzaks (stock size), though, so I'll want to try it again in spring with the stock Dunlops and see what the numbers are.

5. Aircraft account for air pressure by *setting* their barometic altimeters to a corrected figure provided by control towers. If you listen in on aircraft radio traffic, you'll hear the tower read the altimeter setting off as aircraft taxi. That ensures that aircraft have the most accurate altimeter where it really matters -- around the airports where they're getting onto and off of the ground. Once you know the pressure correction at any given time (which is to say, weather condition), barometric altimeters are highly accurate. At the same time, there are a number of altimeter technologies aircraft use. Before GPS, they used radio altimetry (and still do). Nothing more accurate for telling you height above the closest plane-killing solid object. Nowadays, some systems do likewise with laser altimetry. And, of course, when it really counts (instrument approaches in low/no visibility), everythign is done by radio using IFR equipment calculating from a known location, so zero chance for error. Regardless, the point is that nothing with more than one propeller has just one altimeter, but they do all use barometric altimeters as one of them, corrected for local pressure variance.

Again, it's triangulation: no single method of distance measuring is 100% accurate 100% of the time. By using multiple measures, you can better measure the error of your primary measure.

6. If I did get an indication of more than 2% speedometer error at 60 miles per hour, I'd have it in the shop, because that can't be functioning as designed. It's fraud: odometer accuracy matters for warranty enforcement. There *is* a federal spec on odometer accuracy that's tighter than 4%, which is 4,000 miles over 100,000, which is unacceptable for warranty purposes. (Though I haven't bothered to look up the specific spec.) The gov't uses a variety of specs for different purposes, though -- DOT spec for speed measuring commercial traffic, for instance, is 5 miles per hour error at 60 miles per hour, which means they're allowing for up to an 8% error before issuing a speed-based ticket. . . .

--d

 

In every car ( about 7 ) that I have ever owned, the speedometer always reads a tad higher than reality. Chock this up to error on the side of caution... it says I am going 60 MPH when in reality, I'm moving 58.

Also, people usually don't consider the width the the speedometer needle. Which is in most cases, worth about 2 MPH. In the Ford Escape, you are moving 40 MPH only when the entire width of the needle is past the 40 MPH mark. At 60, you are only moving 60 when the entire needle is past the 60 mark.

We had a long discussion over a year ago, and consensus was, you must think of a modern tire as having a perimeter, no circumference or diameter.... because the later two involve circles, and a tire is only a circle when it is off of the ground. When it touches the ground, it always has a flat side. The length of the flat side changes with tire pressure, but the PERIMETER of the tire does not change significantly with tire pressure.

In fact, the perimeter of the stock Ford Tires is measured to be 2100mm at 45 psi. The tires are 2100mm at 35psi, and 2100mm at 25psi. Only at 15psi or less does the tire "relax" enough to measure. At 15psi, the perimeter of the stock tires are 2099mm. 1 mm less.

More significant than tire pressure on ODO readings is tire wear... when the tire actually does lose tread, and therfore, size.

-John

 

Hi everybody!
I'm back now after a very nice four day New Year's weekend.

Okay, since there seems to be some confusion over my original post.
(I'm not the very best explainer of things).
Here's another shot at what I'm trying to say.

I'm not quibbling with anyone's mpg calculations. Your numbers are accurate, I'm not saying they're not.
What I'm saying is that total hybrid fuel efficiency cannot be expressed in mpg terms.

Why is this?
Here's the reason.
Conventional automotive technology increases fuel efficiency by managing fuel use over DISTANCE.
Hybrid technology does this, too, BUT differs fundamentally from conventional technologies in that in addition to managing fuel use over distance, hybrid technology ALSO manages fuel use over TIME.

This is important because total trip elapsed time will vary even though total trip distance may remain constant.
And the critical problem is that not all of this increased fuel efficiency can be expressed in mpg form. It can only be expressed as fuel SAVED over trip TIME. It cannot be expressed only as fuel USED over trip DISTANCE.

Hybrids are never credited fully with their total fuel efficiency because the only (traditional) way we have for measuring fuel efficiency is as gas used over distance; mpg.

My point is that crunching "most accurate" mpg numbers is fine, but it still doesn't yield a fully accurate picture of total hybrid fuel efficiency. And as such, amounts to basically just extra clerical work.

Bottom line: I didn't mean to rain on anyone's parade. If crunching mpg numbers makes you happy, then GO for IT!

 

Have you guys compared the ODO to GPS? i took a trip this past weekend, and the difference between the 2 were only 1.6 miles in 240. So about .6% (fyi- GPS calculated 1.6miles more)

I know the calculations between manual and displayed is off by a couple. But I just use the displayed MPGs as a guideline to see what the current performance is. I think a lot of the difference is because of differnt variables. The only way to be 100% accurate is to fill up at the same station, same pump, same weather conditions, etc. There is going to be a slight variation from station to station and pump to pump.

 

There were significant difference when I first purchased my Prius. The display always exaggerates 5 MPG over manual calculation.

However, after almost a year (Sep 2007 to July 2008) of driving, the display gets closer and closer to my manual calculation. Currently the computer display only exaggerates 1 MPG over my calculation. It's acceptable 2% difference.

 

Absolutely, the media usually bragg about gas saved during driving on highways, or especially on rural routes. For less glamourous conditions, like in a conggested city slower-than-walk traffic, my Prius yields a shabby 29MPG but still produced less pollution that non-hybrid vehicles simply stopped counting MPG (maybe 3 miles per gallon!) This environmental saving as a result of engine shut down during slow traffic seldom gets much deserved credit.

Multiply this factor by millions of city vehicles. You are talking about significant environmental impact which is not credited for, Mr. White Rock! Thanks for bringing up this factor.