Speedo Readings.

[Uncle Bob]

Guys, i'm after a bit of back-up from members with after market sat-nav or GPS speed camera locators.

For about a week i have been using a TALEX camera locator and speed check unit, it gives a constant GPS speed reading which i can only presume is 100% correct?..anyway the RAV4 speedo readings go like this......

RAV4 speedo, ....................... ,GPS true reading,

30mph...............................................27mph

40mph...............................................36mph,

50mph...............................................47mph,

60mph...............................................57mph

70mph...............................................65mph.

It also seems the RAV4's trip meter is way out as a 15.8 miles reading on a trip today,worked out to 21 miles on the GPS unit (yes ,its sort of works in my favour :) ).

Can any of you guys back me up with reading you have from your SAT-NAV units ....cheers.

[Ray99]

Hi Bob

I did a motorway journey recently in a new (56) Yaris and found similar readings. At speedo 70 the GPS (a Navman) said I was doing 68. At speedo 75, GPS said 72, and so on. I think speedo's are set to read a little high to give an impression of speed and to enable you to retain your license for longer. Far better to travel a little slower than to keep attracting the attention of the boys in blue.

[pips]

Hi Guys,

I asked exactly the same question direct to Garmin last year about the accuracy between car speedo and gps. Below is their response (along with my original email).

Cheers, Phil.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hello Phil,

Thank you for contacting Garmin International!

I am happy to help you with this. I would rely on the Gps for speed. You can use statute for miles in the U.S. or Kilometers for the UK. This is your choice.

If you have any additional comments or questions, please do not hesitate to contact us via email or at the number listed below.

Best Regards,

Chris Byer

Product Support Specialist

Garmin International

1200 E. 151st St.

Olathe, KS 66062

For Technical Support please call 1-800-800-1020 (Mon - Fri / 8 - 5 CST)

Fax: 913-440-5488

www.garmin.com

AOL Users please select all text before clicking reply to ensure the original text is included in your reply.

-----Original Message-----

From: Phil Steels []

Sent: Thursday, October 13, 2005 12:21 PM

To: support@garmin.com

Subject: gps accuracy check

I own a GPSMap 60C and have noticed that in any car that I am travelling in, there is always a descrepancy between the indicated speed on the gps and the speed indicated on the car.

car 40mph gps 37.1mph

car 60mph gps 56.6mph

Can you tell me whether the gps is 100% accurate on the speed so I know which one to rely upon when driving.

Also, something that I thought of, does the gps use US miles which I believe are slightly different than UK miles.

Regards,

Phil Steels

[chatman]

Guys, i'm after a bit of back-up from members with after market sat-nav or GPS speed camera locators.

For about a week i have been using a TALEX camera locator and speed check unit, it gives a constant GPS speed reading which i can only presume is 100% correct?..anyway the RAV4 speedo readings go like this......

RAV4 speedo, ....................... ,GPS true reading,

30mph...............................................27mph

40mph...............................................36mph,

50mph...............................................47mph,

60mph...............................................57mph

70mph...............................................65mph.

It also seems the RAV4's trip meter is way out as a 15.8 miles reading on a trip today,worked out to 21 miles on the GPS unit (yes ,its sort of works in my favour :) ).

Can any of you guys back me up with reading you have from your SAT-NAV units ....cheers.

Bob,

My TomTom Nav 5 reads about the same as your's. I use this to judge my speed as the speedo reads slightly out....and also I have the camera locations POI.

[anchorman]

Guys, i'm after a bit of back-up from members with after market sat-nav or GPS speed camera locators.

For about a week i have been using a TALEX camera locator and speed check unit, it gives a constant GPS speed reading which i can only presume is 100% correct?..anyway the RAV4 speedo readings go like this......

RAV4 speedo, ....................... ,GPS true reading,

30mph...............................................27mph

40mph...............................................36mph,

50mph...............................................47mph,

60mph...............................................57mph

70mph...............................................65mph.

It also seems the RAV4's trip meter is way out as a 15.8 miles reading on a trip today,worked out to 21 miles on the GPS unit (yes ,its sort of works in my favour :) ).

Can any of you guys back me up with reading you have from your SAT-NAV units ....cheers.

Costruction and Use regulations (EEC Type Approval) says that and discrepancy in the speedometer of a car must read high. Occasionally, car magazines will show actual speed with indicated speed and they usually tally up quite well with what you have shown. The speedo is driven from a worm gear in the final drive and then converted to an electronic signal that is converted back in the speedo head. Toyota will have chosen the nearest ratio that puts the reading just high of accurate (knowing it is near impossible to get it perfect - dont forget the readings alter as the tyres wear and the number of revolutions increase over a given distance, albiet slight). The only real way to calibrate your speedo is to time yourself over a fixed distance - 1 mile = 60 seconds at 60mph. If you know a freindly road traffic policeman I would have thought he should have a calibrated speedo or optical measuring device. Don't worry he won't nick you for having a duff speedo because they are all reading high!

I don't know enough about the accuracy of a GPS unit to comment but looking at what you have shown I would suggest that they are pretty near the mark.

Regards

[mlp]

Hi all

i am running TomTom nav3 with my Rav4 5Door Auto and

my readings are similar to your's Bob

in fact my previous motor an old VW Passat had the same

sort of readings using the same GPS

Bob judging by your short journey we must be doing more

miles than we thought, can you verify your findings as it

would make a big difference to the MPG, or am I dreaming?

Cheers

marshall

[anchorman]

Hi all

i am running TomTom nav3 with my Rav4 5Door Auto and

my readings are similar to your's Bob

in fact my previous motor an old VW Passat had the same

sort of readings using the same GPS

Bob judging by your short journey we must be doing more

miles than we thought, can you verify your findings as it

would make a big difference to the MPG, or am I dreaming?

Cheers

marshall

Hi Marshall

Sorry to interfere but I think you are really doing less miles than you thought. (Your speedo is telling you artificially high). You will be getting less to the gallon than the trip computer is telling you but it's all relative.

Regards

anchorman

[Uncle Bob]

Hi all

i am running TomTom nav3 with my Rav4 5Door Auto and

my readings are similar to your's Bob

in fact my previous motor an old VW Passat had the same

sort of readings using the same GPS

Bob judging by your short journey we must be doing more

miles than we thought, can you verify your findings as it

would make a big difference to the MPG, or am I dreaming?

Cheers

marshall

I see were you're coming from :) , if the trip meter is reading 25% less miles than my GPS reads, i could be getting 25% more miles to the gallon as i'm taking the trip meter as correct .

[pips]

Hi all

i am running TomTom nav3 with my Rav4 5Door Auto and

my readings are similar to your's Bob

in fact my previous motor an old VW Passat had the same

sort of readings using the same GPS

Bob judging by your short journey we must be doing more

miles than we thought, can you verify your findings as it

would make a big difference to the MPG, or am I dreaming?

Cheers

marshall

I see were you're coming from :) , if the trip meter is reading 25% less miles than my GPS reads, i could be getting 25% more miles to the gallon as i'm taking the trip meter as correct .

Interest thought.... I wonder if you could go through a speed camera at the speedo reading of 37.5mph (30mph + 25%) and not get a ticket? arguing the point that the GPS reading was 30mph?

Phil

[Uncle Bob]

As a point of interest, those flashing LED speed signs you get on lamp posts ,give an identical reading to my GPS, so they must be calibrated the same or similar to speed cameras, so i feel you could drive 2-3 mph faster using a GPS unit, towards a speed gun policeman ...if you have the bottle :D :) .

[anchorman]

As a point of interest, those flashing LED speed signs you get on lamp posts ,give an identical reading to my GPS, so they must be calibrated the same or similar to speed cameras, so i feel you could drive 2-3 mph faster using a GPS unit, towards a speed gun policeman ...if you have the bottle :D :) .

In the words of Dirty Harry, "well punk, do ya feel lucky?" B)

They tend to set the speed at about 36 for giving you a ticket to allow a couple of mph for speedo inaccuracy and a couple for good measure. My Mrs got nicked for doing 37 and they argued that she exceeded the margin.

[tufty9]

Bob

I've got a Talex unit (the original not the lite version) and the discrepancy in speed is exactly the same as yours.

Out of interest when you update the Talex unit using a PC & USB port, does yours take about an hour to update? It says on the update page it should only take 5 minutes, but the first update mine took 90 minutes and 2nd & 3rd times about an hour - So just wondering if it's either my PC (I have broadband & windows XP) or if everyone's take an age to update.

[Uncle Bob]

Bob

I've got a Talex unit (the original not the lite version) and the discrepancy in speed is exactly the same as yours.

Out of interest when you update the Talex unit using a PC & USB port, does yours take about an hour to update? It says on the update page it should only take 5 minutes, but the first update mine took 90 minutes and 2nd & 3rd times about an hour - So just wondering if it's either my PC (I have broadband & windows XP) or if everyone's take an age to update.

I have the TALEX unit (not the lite ) and it took me a few goes to get the downloads to work , but the downloads only seem to take a few minutes to do in the end.

[themanhimself]

just wondering does the gradient of road generate a difference. i.e. if you go 1 mile up a steep hill the distance will be greater than the horizontal shift. i.e. does a gps take into account vertical movement when calculating speed I don't know but this may account for difference

[anchorman]

just wondering does the gradient of road generate a difference. i.e. if you go 1 mile up a steep hill the distance will be greater than the horizontal shift. i.e. does a gps take into account vertical movement when calculating speed I don't know but this may account for difference

I don't profess to be an expert but I think because the GPS uses more than 3 satellites (I think 5 to 7 is not uncommon) that it can triangulate your position in 3 dimensions. In other words if you were to climb vertically (or fall off a cliff!) it will know the distances involved.

Very early versions that used only one satellite could not triangulate but they were used by mariners where hills are not a problem.

[Uncle Bob]

just wondering does the gradient of road generate a difference. i.e. if you go 1 mile up a steep hill the distance will be greater than the horizontal shift. i.e. does a gps take into account vertical movement when calculating speed I don't know but this may account for difference

I don't profess to be an expert but I think because the GPS uses more than 3 satellites (I think 5 to 7 is not uncommon) that it can triangulate your position in 3 dimensions. In other words if you were to climb vertically (or fall off a cliff!) it will know the distances involved.

Very early versions that used only one satellite could not triangulate but they were used by mariners where hills are not a problem.

Yes i would think the more satellites watching you, the more accurate the reading (up or down hill ), i've had upto 9 tracking me at any one time, thats in good weather conditions :) .

[pips]

when i was in the usa last month, i was using my sat nav with an external aerial ('cos the windshield antenna blocked the signal) - we were travelling near Pearsonville and i got full signal strength on all 12 satellites!! first time i ever had 100% accuracy too!

Phil

[gcparris]

Lol - that is quite impressive!

when i was in the usa last month, i was using my sat nav with an external aerial ('cos the windshield antenna blocked the signal) - we were travelling near Pearsonville and i got full signal strength on all 12 satellites!! first time i ever had 100% accuracy too!

Phil

[JollyG]

when i was in the usa last month, i was using my sat nav with an external aerial ('cos the windshield antenna blocked the signal) - we were travelling near Pearsonville and i got full signal strength on all 12 satellites!! first time i ever had 100% accuracy too!

Phil

Are those TIE fighters? :)

Obviously the force was with you...

[pips]

when i was in the usa last month, i was using my sat nav with an external aerial ('cos the windshield antenna blocked the signal) - we were travelling near Pearsonville and i got full signal strength on all 12 satellites!! first time i ever had 100% accuracy too!

Phil

Are those TIE fighters? :)

Obviously the force was with you...

If I'd been on Nevada Route 375 at the time (The Extra Terestrial Highway) - I could well have been a bit worried..........

[Uncle Bob]

Can any of you guys confirm that your trip meter reads less that you GPS units readout (any Toyota model ), if so, how much by ?....cheers :) .

[pips]

Can any of you guys confirm that your trip meter reads less that you GPS units readout (any Toyota model ), if so, how much by ?....cheers :) .

sorry Bob, think we all got a bit carried off track there!!!! will check it out over the weekend and post the figures for you.

cheers, phil

[Uncle Bob]

Can any of you guys confirm that your trip meter reads less that you GPS units readout (any Toyota model ), if so, how much by ?....cheers :) .

sorry Bob, think we all got a bit carried off track there!!!! will check it out over the weekend and post the figures for you.

cheers, phil

Never thought of the thread as going off topic...as it was still GPS related subject .

[Sp33dy1969]

Just to confirm that my speedo is indeed out too...

I've put the Toyota Sat Nav (full version) into diag mode and it's on screen speed reading is about 6 mph less than the speedo at 70 mph. At lower speeds it's not as far out. 30 on the speedo is about 27 on the nav.

regards

Steve.

[anchorman]

Can any of you guys confirm that your trip meter reads less that you GPS units readout (any Toyota model ), if so, how much by ?....cheers :) .

It's relative Bob. If the speedo is 10% out the mileage will be so it varies from car to car and how much tyre wear there is. I did a quick sum to see what effect tyre wear has. My wheel is about 28 inches in diameter. So 28 times 3.142/12 means the car moves 7.33 feet per revolution so the wheel revolves 720 times per mile. I reckon that the max tyre wear is about 9mm or 3/8 inch which is 3/4 inch on diameter so they only cover 7.13 per revolution. It doesn't sound much but new tyre will cover about 145 feet more per mile for the same number of turns! It means old tyres have to revovle nearly 20 times more per mile (to cover the same distance). You can safely say 2.7% or 3% for round numbers variation in speedo only due to tyre wear and then there is the positive descrepancy that is built in.

Surprising isn't it?

Regards