Mpg(again)

[gunner1]

If I need to try for ACTUAL 30 mpg what should my OBC reading be on a GX Auto ?

Any simple way to check ACTUAL mpg V OBC ?

[bothwell_buyer]

If I need to try for ACTUAL 30 mpg what should my OBC reading be on a GX Auto ?

Any simple way to check ACTUAL mpg V OBC ?

yOU COULD TRY ATTACHING ONE OF THOSE THINGS WITH ONE WHEEL WHICH THEY USE TO MEASURE ROAD DISTANCES, TO THE REAR WHEEL ARCH; AND THEN USE A CALIBRATED DIPSTICK TO MEASURE THE PETROL (damn these caps lock buttons)

[Hoovie]

you need to reset your OBC on each fill-up, then on the next fill-up, note the average OBC MPG against the REAL MPG you have got from working out the miles done against fuel put in.

Then over a few fills, you will see if there is any kind of consistant pattern.

This is my variance from March to September, noted on each tank fill

Actual v OBC

20%

20%

25%

18%

28%

22%

24%

22%

21%

25%

17%

24%

22%

22%

So anywhere from 17% to 28% optimisitic on the OBC, average amount out? maybe 22%?

I used to try and target an average 40 MPG on the OBC display, reckoning I would be doing maybe 32MPG at least in real life, but the Toyota OBC seem to very inconsistant between cars, so as a variation to YMMV, YOUR Mileage WILL Vary!

[anchorman]

All your calcs are only as good as the milometer and as the speedo reads high with new tyres it will so with the milo - even more with worn tyres. You need a calibrated amount of fuel and a calibrated track with acurate distance markers.

I wouldn't worry too much about it!!!

[Hoovie]

Using a GPS is also a handy way to check the accuracy of your odometer as well as the speedo accuracy.

[local hero]

Using a GPS is also a handy way to check the accuracy of your odometer as well as the speedo accuracy.

I Thought about this, if you went up a hill 1/2 mile high + 1/2 m down the other side, but you had only travelled 1/2 mile in distance , In other words a perfect tri-angle, Your ground position has only altered by 1/2 m whereas in truth you have driven 1mile, so surely your gps would be 50% inaccurate, come on you cleverer people, am i right? :D :D

[MarsKy]

Using a GPS is also a handy way to check the accuracy of your odometer as well as the speedo accuracy.

I Thought about this, if you went up a hill 1/2 mile high + 1/2 m down the other side, but you had only travelled 1/2 mile in distance , In other words a perfect tri-angle, Your ground position has only altered by 1/2 m whereas in truth you have driven 1mile, so surely your gps would be 50% inaccurate, come on you cleverer people, am i right? :D :D

Distance and time - you're assuming that the GPS is "sampling" at an absolute distance of 1/2m during the time of travel, whereas it will be sampling and calculating a relative distance against time continuously.

[Hoovie]

Using a GPS is also a handy way to check the accuracy of your odometer as well as the speedo accuracy.

I Thought about this, if you went up a hill 1/2 mile high + 1/2 m down the other side, but you had only travelled 1/2 mile in distance , In other words a perfect tri-angle, Your ground position has only altered by 1/2 m whereas in truth you have driven 1 mile, so surely your gps would be 50% inaccurate, come on you cleverer people, am i right? :D :D

Interesting point, Mr Hero

I think you are correct in principle, in fact, but the margin in error in a GPS is probably counterbalanced (or made worse!!) by the variance in fuel on a fillup depending on the ambient temperature.

If you could point me to the road that has a 60 Deg incline for 1/2 Mile (could be a struggle to get up there!) and then 60 Deg decline (woah, that would be a ride and a half!), I will try it out.

In fact, I may well try this out on my handheld GPS at Jacobs Ladder down the road, as that has similar characteristics to the one you picture, albeit a few yards ground distance from start to finish rather then a 1/2 mile

[Hoovie]

Using a GPS is also a handy way to check the accuracy of your odometer as well as the speedo accuracy.

I Thought about this, if you went up a hill 1/2 mile high + 1/2 m down the other side, but you had only travelled 1/2 mile in distance , In other words a perfect tri-angle, Your ground position has only altered by 1/2 m whereas in truth you have driven 1mile, so surely your gps would be 50% inaccurate, come on you cleverer people, am i right? :D :D

Distance and time - you're assuming that the GPS is "sampling" at an absolute distance of 1/2m during the time of travel, whereas it will be sampling and calculating a relative distance against time continuously.

That is another interesting point :D

For the Odometer bit, time is not relevant and the distance probably WILL be incorrectly shown - a GPS works out distance based on triangulation positioning from one sample instance to the next instance

Say that the sampling is every 1 minute for example (for ease of the numbers) and that the car is going up this hill at a rate of 100 foot a minute.

For every 100 foot it has travelled, the 'flat' ground distance is only 50 foot and the GPS will only have recorded 50 foot as the distance travelled, based on the triangualation positions.

On the Speed bit, in this situation, the speed shown by the GPS will actually be lower by 50% then the cars speed - as in this 1 minute, the car will have travelled at a rate of 100ft/minute, but the GPS will have calculated a speed of 50ft/Minute.

Interesting discussion for a Thursday morning :)

[dave.m]

Still confused ? see link :D

http://www.pocketgpsworld.com/howgpsworks.php

[shcm]

For every 100 foot it has travelled, the 'flat' ground distance is only 50 foot and the GPS will only have recorded 50 foot as the distance travelled, based on the triangualation positions.

Sorry Hoovie, I have to disagree.

Try not to think in 2D. GPS is a 3D system, (yes, the altitude bit is no where near as accurate as the lat/long bit) although of course it can work with only a 2D fix.

Typically the fix update of off the shelf GPS receivers, is 1 second. I would hope the distance calculation is based on the length of the line in 3D space between the two fix points taken 1 second apart. The total distance being the sum of all these little line segments.

If you jumped out of a stationary hot air balloon with a GPS receiver, by your argument, wouldn't it read 0 speed on the way down? I think it might show more than that, but I have to confess, I haven't tried it. :D

Cheers

[Parts-King]

Using a GPS is also a handy way to check the accuracy of your odometer as well as the speedo accuracy.

I Thought about this, if you went up a hill 1/2 mile high + 1/2 m down the other side, but you had only travelled 1/2 mile in distance , In other words a perfect tri-angle, Your ground position has only altered by 1/2 m whereas in truth you have driven 1mile, so surely your gps would be 50% inaccurate, come on you cleverer people, am i right? :D :D

Distance and time - you're assuming that the GPS is "sampling" at an absolute distance of 1/2m during the time of travel, whereas it will be sampling and calculating a relative distance against time continuously.

That is another interesting point :D

For the Odometer bit, time is not relevant and the distance probably WILL be incorrectly shown - a GPS works out distance based on triangulation positioning from one sample instance to the next instance

Say that the sampling is every 1 minute for example (for ease of the numbers) and that the car is going up this hill at a rate of 100 foot a minute.

For every 100 foot it has travelled, the 'flat' ground distance is only 50 foot and the GPS will only have recorded 50 foot as the distance travelled, based on the triangualation positions.

On the Speed bit, in this situation, the speed shown by the GPS will actually be lower by 50% then the cars speed - as in this 1 minute, the car will have travelled at a rate of 100ft/minute, but the GPS will have calculated a speed of 50ft/Minute.

Interesting discussion for a Thursday morning :)

I'm lost...........literally

More importantly..............What time does the pie van arrive?

Kingo

[hertsnminds]

Try not to think in 2D. GPS is a 3D system

Remember why the US gave the World GPS in the first place, to target weapons. A system that only works in 2D would not be very good if it did not take account of travel on the Z axis (X and Y axis only).

It might seem that three satellites would be enough to solve for a position, since space has three dimensions. However, a three-satellite solution requires the time be known to a nanosecond or so, far better than any non-laboratory clock can provide. Using four or more satellites allows the receiver to solve for time as well as geographical position, eliminating the need for a very accurate clock. In other words, the receiver uses four measurements to solve for four variables: x, y, z, and t.

Source http://en.wikipedia.org/wiki/Global_Positioning_System

[Hoovie]

For every 100 foot it has travelled, the 'flat' ground distance is only 50 foot and the GPS will only have recorded 50 foot as the distance travelled, based on the triangualation positions.

Sorry Hoovie, I have to disagree.

Try not to think in 2D. GPS is a 3D system, (yes, the altitude bit is no where near as accurate as the lat/long bit) although of course it can work with only a 2D fix.

Typically the fix update of off the shelf GPS receivers, is 1 second. I would hope the distance calculation is based on the length of the line in 3D space between the two fix points taken 1 second apart. The total distance being the sum of all these little line segments.

If you jumped out of a stationary hot air balloon with a GPS receiver, by your argument, wouldn't it read 0 speed on the way down? I think it might show more than that, but I have to confess, I haven't tried it. :D

Cheers

I do get the 3D bit ;) my sampling example of 1 minute was to make it easy for the numbers :)

As you say, the level of accuracy with regards to changes in Altitude is very poor in comparision to the accuracy of changes in Latitude/Longitude. on my HandHand GPS, it actually bases the Altitude on a combination of changes in Air Pressure then satellite data as a better indication, even though air pressure varies in the same place anyway.

Hard to evaluate what really happens as the amount of vertical movement is so small compared to the amount of horizontal movement is that there may not be enough granularity displayed to know.

I will do the "jacobs ladder" test and look at the trip data - that will actually give me instanteous data of a 3D position in the 'journey'

Re the GPS Speed dropping out of a balloon - Actually, I would not be too sure that it will not show Zero!! the velocity should be 9.8M/s if I recall correctly?

Also, Military GPS units are far more accurate then Commercial units, so the position on the vertical axis will be much more exacting

Interesting

so .... The receiver can utilize multidimensional Newton-Raphson calculations

together with

Yeah

:D

[dave.m]

As my old friend Jean Luc Picard would say, aha space time continuum see link -

Or was it Pythagoras

http://en.wikipedia.org/wiki/Spacetime

[shcm]

Re the GPS Speed dropping out of a balloon - Actually, I would not be too sure that it will not show Zero!! the velocity should be 9.8M/s if I recall correctly?

That's what I said, it's unlikely to show zero, there is still movement, but not in the horizontal plane.

Terminal velocity surely depends on drag on the body? Accel due to gravity is approx 9.8 m/s^2, is I think what you mean.

I'd have a lie down mate.

[Hoovie]

Re the GPS Speed dropping out of a balloon - Actually, I would not be too sure that it will not show Zero!! the velocity should be 9.8M/s if I recall correctly?

That's what I said, it's unlikely to show zero, there is still movement, but not in the horizontal plane.

Terminal velocity surely depends on drag on the body? Accel due to gravity is approx 9.8 m/s^2, is I think what you mean.

I'd have a lie down mate.

Ah, you missed my double-negative in the response :D

"I would not be too sure that it will not show Zero" ;) - I should have put an 'although' between the two sentences.

Yup - it is acceleration, not velocity I was trying to dredge from the recesses of the brain (it is a long time since I did any physics or complex equations :( )

But the one question that is still outstanding .....

Does this discussion answer the original question, or have we ever so slightly, minisculey veered off-topic a tad, or a smidge, or even 1/4th of an Iota? B)

[jonnyspin]

most interesting thing i've read day! - thanks

[hertsnminds]

Just go easy on the equations and proof's or we could end up with a Babel Fish scenario...

http://www.youtube.com/watch?v=dcncPpQ8loA

[Hoovie]

All I can say is :fishyface:

[local hero]

Well i posted this before going to work this morning, ive just got in, read this and i'm going back out again, driving a cab is so much easier

Holy Mary mother of god, what on earth have i started, and Ive just become a Guru member, :lol2: :lol2: :lol2: :lol2:

(Local Guru)

[Parts-King]

All these equations flying around is fascinating, but I forgot to mention one thing.............

The pie van never got here yesterday, hope they switch the Sat Nav off today

Kingo

[Jsy_Rav4]

GPS uses up to 12 satellites to triangulate (I seem to recall) and needs around 5 to get an accurate position... it should be able to work out distance taking into account inclines etc and I find is generally far more accurate than in car speedo readings - can't verify distance easily but assuming the unit is on at all times when the car is moving (allowing up to 20/30 seconds for getting a "first fix") the only time it would be out is a lot of distance travelled in car parks, tunnels etc.

The data from my head unit (Kenwood DNX-5220BT) seems to be amazingly accurate when travelling towards speed indicators mounted on lamp posts etc in various locations over here - even at low speeds (we have a few in "green lanes" over here in Jersey where the max road limit is 15mph !)

Even considering our max limit is 40mph and most roads are 30mph, I'm getting around 32 mph from my 2.0l diesel (currently diesel is around 85p / litre at one forecourt here - and no supermarket "weasel pee" on the Island either).

[anchorman]

Well i posted this before going to work this morning, ive just got in, read this and i'm going back out again, driving a cab is so much easier

Holy Mary mother of god, what on earth have i started, and Ive just become a Guru member, :lol2: :lol2: :lol2: :lol2:

(Local Guru)

This is all because you think there is only one satellite and it is always directly above your car Stewpot.

There is actually but that belongs to Greater Manchester Police Taxi Crime unit!!! :P