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Old 12-31-2013, 02:20 PM   #1
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Odometer verses GPS

On our last trip (2000 miles), I kept track of the MPG. Using the speedometer/odometer it clocked in at 8 MPG, but using the mileage recorded on my RM GPS it was 7.2. Watching the highway markers as reference, it appears the GPS is more accurate than the speedo. I know the factory speedo is never accurate. I was surprised just how far off it is.

What are your thoughts.... Is the GPS more accurate when it comes to miles traveled?
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Old 12-31-2013, 02:38 PM   #2
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10% is probably the normal variance. More importantly is the speedometer running the same 10-11% high? You might feel pretty good staying between 55 and 60 and wonder why you get dirty looks when you think you have stayed at or above 55 and it really was between 50 and 55.
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Old 12-31-2013, 02:46 PM   #3
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I know that the speed on the GPS is based on time between two points. The speedo is real time. I noticed just a small difference between GPS and speedo as far as speed.. maybe 3 MPH. Speedo reads faster, as it should.
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Old 12-31-2013, 03:52 PM   #4
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HAH! For once, a tech question on the forums that I can answer with authority!! (I am a Space Systems Analyst...yeppers a real live rocket scientist )

In almost every case, with a modern GPS unit (>2 years old), the GPS will beat out the odometer (or speedometer). This especially holds true with a older vehicle, one with worn tires, or a vehicle that uses a mechanical odometer/speedometer!

So how doe it work? Glad you asked (as he takes a pointer out of his pocket protector )

No matter where you are, at least four GPS satellites are able to be acquired by your GPS unit at any given time. Each satellite transmits data packets to your GPS recever (technically we refer to them as PNT packets – Position, Timing & Navigation).

Transmission and reception are constant (signals travel at light speed), thereby allowing the GPS receiver to calculate how far away each satellite is based on how long it took for the PNT packet to arrive (there are cross-comparisons but that is TOO technical! ).

Now that the GPS recieverin your vehicle has this information, it takes the data from the 3 strongest satellite signals and geolocates your position using a process called trilateration.

Here is a quick and dirty explanation of trilateration: Imagine you are standing somewhere on Earth with three satellites in the sky above you. Each satellite has a certain coverage area. If you happen know how far away you are from satellite A, then you know you must be located somewhere in Satellite A’s coverage area. If you do the same for satellites B and C, you can work out your location by seeing where the three coverage areas intersect (think 3-way VENN diagram here)

This is pretty much what your GPS receiver does, (although it does it in three dimensions, using overlapping spheres rather than circles).

So, howthehelldoes a GPS know what time it is (including auto adjusting for time zones?) GPS satellites have on board atomic clocks to keep accurate time (the T or timing part of the PNT packet

General and Special Relativity predict that differences will appear between these clocks and an identical clock on Earth:

General Relativity predicts that time will appear to run slower under stronger gravitational pull – the clocks on board the satellites will therefore seem to run faster than a clock on Earth.

Special Relativity predicts that because the satellites’ clocks are moving relative to a clock on Earth, they will appear to run slower.

The GPS network makes allowances for these effects – which is SOLID proof that all that Relativity mumbo-jumbo you learned in High School or College physics has a real world impact!.

I stole the graphic below from a GPS briefing I gave a while back – hope this helps you visualize what I wrote above!

(Hands out aspirin to those who have headaches by now
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Old 12-31-2013, 03:58 PM   #5
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So, in a nutshell, GPS ruined my MPG


J/K... Thanks Bob! I suspected the GPS was more accurate.
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Old 12-31-2013, 04:03 PM   #6
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On a related note, when I first bought my Ford Explorer, I calculated my MPG using the odometer. I started bragging to all my folks that I was getting 26 MPG. The EPA estimate was 19.

Then the GPS was used..... You got it... averaged 17.5.

Can't help but wonder if the speedo is not accurate for a reason.... just sayin'
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Old 12-31-2013, 05:51 PM   #7
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Quote:
Originally Posted by MrJoe View Post
So, in a nutshell, GPS ruined my MPG


J/K... Thanks Bob! I suspected the GPS was more accurate.
LOL!! Well sorta - maybe your lead foot is a covariate? :roll eyes:

(J/K too )
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Old 12-31-2013, 06:00 PM   #8
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Quote:
Originally Posted by MrJoe View Post
On a related note, when I first bought my Ford Explorer, I calculated my MPG using the odometer. I started bragging to all my folks that I was getting 26 MPG. The EPA estimate was 19.

Then the GPS was used..... You got it... averaged 17.5.

Can't help but wonder if the speedo is not accurate for a reason.... just sayin'
Possibly Joe - the question is what type of system? The new systems (2012+) are pretty accurate and have completely abandoned the old mechanical ones that vary based on tread wear, wheel size and other mechanical factors)

The bottom line is there are no real super-precise methods of assessing gas milage (even the old fill er up, drive EXACTLY 100 miles, fill 'er up, do the math to calculate gas milage has averaging problems - but if you are really OC about finding your milage you could do THIS as a comparison test ...although the travel computer in most modern GPSs will probably be just as good - perhaps a bit better)
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Old 12-31-2013, 06:09 PM   #9
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Thanks Bob, wow I'm impressed. I tried to think '3-way VENN diagram' there for a short second but my brain rejected the notion, it does that frequently since I retired but the graphic allowed it to eaz on through.
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Old 12-31-2013, 06:55 PM   #10
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Quote:
Originally Posted by HSVBamaBob View Post
HAH! For once, a tech question on the forums that I can answer with authority!! (I am a Space Systems Analyst...yeppers a real live rocket scientist )

In almost every case, with a modern GPS unit (>2 years old), the GPS will beat out the odometer (or speedometer). This especially holds true with a older vehicle, one with worn tires, or a vehicle that uses a mechanical odometer/speedometer!

So how doe it work? Glad you asked (as he takes a pointer out of his pocket protector)

No matter where you are, at least four GPS satellites are able to be acquired by your GPS unit at any given time. Each satellite transmits data packets to your GPS recever (technically we refer to them as PNT packets – Position, Timing & Navigation).

Transmission and reception are constant (signals travel at light speed), thereby allowing the GPS receiver to calculate how far away each satellite is based on how long it took for the PNT packet to arrive (there are cross-comparisons but that is TOO technical!).

Now that the GPS recieverin your vehicle has this information, it takes the data from the 3 strongest satellite signals and geolocates your position using a process called trilateration.

Here is a quick and dirty explanation of trilateration: Imagine you are standing somewhere on Earth with three satellites in the sky above you. Each satellite has a certain coverage area. If you happen know how far away you are from satellite A, then you know you must be located somewhere in Satellite A’s coverage area. If you do the same for satellites B and C, you can work out your location by seeing where the three coverage areas intersect (think 3-way VENN diagram here)

This is pretty much what your GPS receiver does, (although it does it in three dimensions, using overlapping spheres rather than circles).

So, howthehelldoes a GPS know what time it is (including auto adjusting for time zones?) GPS satellites have on board atomic clocks to keep accurate time (the T or timing part of the PNT packet

General and Special Relativity predict that differences will appear between these clocks and an identical clock on Earth:

General Relativity predicts that time will appear to run slower under stronger gravitational pull – the clocks on board the satellites will therefore seem to run faster than a clock on Earth.

Special Relativity predicts that because the satellites’ clocks are moving relative to a clock on Earth, they will appear to run slower.

The GPS network makes allowances for these effects – which is SOLID proof that all that Relativity mumbo-jumbo you learned in High School or College physics has a real world impact!.

I stole the graphic below from a GPS briefing I gave a while back – hope this helps you visualize what I wrote above!

(Hands out aspirin to those who have headaches by now
Yeah, what Bob said...I was gonna' say the same thing.
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