Which do I trust? (speedometer or GPS)

 

My nuvi 650 tells me I am going one speed, my spedometer tells me I'm going a few MPH slower. At 70, by the nuvi, it says almost 74 on the speedometer.... I'm inclined to think the nuvi has it right..

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John - with a Garmin 650 and a 750
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Which do I trust?

I get the same results with my Nuvi350. I would go with your spedometer. I'm no expert, but I'm guessing it has to do with the accuracy of the GPS (usually around 10 - 20 feet). Would this account for the discrepancy?

--
I have seen the future and it is now!

Sorry, I disagree and here

Sorry, I disagree and here is why. I have shot my car with radar, it said that I was going 60. My speedometer said 60 and my GPS said 60. In other words, everyone agreed that I was going 60. Now, every rental car that I have used lately said that I was going 2 to 3 mph faster then what my GPS said.

Therefore, Since my GPS was correct with my car and radar, it should be right with the rental, but it isn't so the rental is off.

Btw, I tested myself going 70, 80, and 90. At 70, my car said that I was going 70, but my GPSr and Radar gun said 68. So I trust my GPSr/radar gun more so then the car's speedometer (two over one). The same was for 80 and 90 mph as well.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

.

GPS receivers are definitely more accurate that vehicle speedometers.

I agree with AsianFire

In all cases where I have run into one of those MPH
indicators along the roadway it has agreed with my GPSr. My van speedometer indicates about 3-4 mph faster than the radar unit at 70 MPH. Also speedometer accuracy will vary with tire pressure and tire wear. Not by a great amount but 2-3 % over the life of the tires.
I also have calculated my speed using the mile markers along the highway (before I owned GPS) and that also showed my speedometer reading high.
I go by my GPSr.
My observations !!!!!!!

--
MrKenFL- "Money can't buy you happiness .. But it does bring you a more pleasant form of misery." NUVI 260, Nuvi 1490LMT & Nuvi 2595LMT all with 2014.4 maps !

it really boils down to,

it really boils down to, What does the policeman's radar unit say that you were going......lol.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

speedometer calibration

some states have laws that allow 2-3% speed rating before writing a ticket. this is due to the fact that the speedometer calibration is not always accurate.

I could be wrong but when you change your tires to a smaller or bigger tire, I would think that your speedometer would need to be re calibrated. this would affect the revolution of the tires, causing more or less distance...??!!!??

And where can

asianfire wrote:

Btw, I tested myself going 70, 80, and 90. At 70, my car said that I was going 70, but my GPSr and Radar gun said 68. So I trust my GPSr/radar gun more so then the car's speedometer (two over one). The same was for 80 and 90 mph as well.

I take my vet for a ride at 90 mph?
And not have the blue lights behind me? cool

--
Computers are useless. They can only give you answers. - Pablo Picasso (Bob - Garmin c530, eTrex Vista HCx)

My GPS is always 2 to 3mph

My GPS is always 2 to 3mph slower in reading compared to my cars speedometer. You know which one I'm going with wink.

--
nüvi 3590LMT "always backup your files"

Speedometers depend on..

Speedometers are analog devices that depend on the revolutions your tires are making ( I know they don't connect directly to the tires, so please don't call me on that). As your tires wear down, they have to turn faster in order for you to maintain the same speed, so it will look like your speedometer will say you're going faster than you really are. Like all instruments, speedometers have a range of accuracy, and auto manufacturers would rather err on the side of caution, so normally, they are factory set a little on the fast side... They don't want to be pulled into court defending your speeding ticket.

The next time you're on an interstate, drive at a steady rate and time yourself between mile markers. That always works, as do working GPS units.

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cheweez78 is right!

cheweez78 wrote:

some states have laws that allow 2-3% speed rating before writing a ticket. this is due to the fact that the speedometer calibration is not always accurate.

I could be wrong but when you change your tires to a smaller or bigger tire, I would think that your speedometer would need to be re calibrated. this would affect the revolution of the tires, causing more or less distance...??!!!??

The difference in the calibration of the speedometer with stock tires greatly changes when you change the size of your tires, whether the tire is bigger or smaller. You have to have your speedometer recalculated to the size of the tire that you use! Nothing changes the GPS! I,ve been told that even normal wear of the tires effect the speedometer!

--
Richie

Which do I trust? (speedometer or GPS)

Trust your GPS The satellite is the Most accurate of anything That is also Per The OH State Hwy Patrol and The DOT Lot Of Truckers ask that same Question

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Auggie SP2720 , SP C330, Nuvi 650, Nuvi 785T America Moves By Truck

I think the GPS is less

I think the GPS is less reliable, and my reasoning is because when I look at what it says my max speed is, it says 143KM/H, which I haven't been up to that speed in at least 10 years, I don't go past 115KM/H.
I'm not sure why its saying that, but i ignore it, i know i'm not going that fast.

Reset that Max speed

And trust your GPS - it is very accurate.

As to why your unit says that for a max speed - who knows...could be lots of reasons.

--
*Keith* MacBook Pro *wifi iPad(2012) w/BadElf GPS & iPhone6 + Navigon*

Who Knows

kch50428 wrote:

And trust your GPS - it is very accurate.

As to why your unit says that for a max speed - who knows...could be lots of reasons.

I once heard that the government will randomly throw out an error. That has been 10+ years ago though.

--
Garmin nuvi 2455LMT (wife uses nuvi 255w) (sold C330)

.

Selective Availability (SA) was turned off on May 1, 2000. And on September 19, 2007, the government announced that they weren't going to purchase anymore satellites capable of SA.

yes tires can make a difference

cheweez78 wrote:

some states have laws that allow 2-3% speed rating before writing a ticket. this is due to the fact that the speedometer calibration is not always accurate.

I could be wrong but when you change your tires to a smaller or bigger tire, I would think that your speedometer would need to be re calibrated. this would affect the revolution of the tires, causing more or less distance...??!!!??

Yes, tires can make a BIG difference.

--
Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

Vets

Summermug wrote:
asianfire wrote:

Btw, I tested myself going 70, 80, and 90. At 70, my car said that I was going 70, but my GPSr and Radar gun said 68. So I trust my GPSr/radar gun more so then the car's speedometer (two over one). The same was for 80 and 90 mph as well.

I take my vet for a ride at 90 mph?
And not have the blue lights behind me? cool

They may be chick magnets, but they are cop magnets as well...lol. Still would love one.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

My nuvi 200, digital speedometer, and ...

My nuvi 200, digital speedometer, and the two roadside radar signs I pass every day usually agree exactly, but never vary by more than 1 mph (probably rounding error). My tires only have 10K miles on them. As they wear my speedometer should start reading high. I will be curious to see how high before it is time to change them.

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><> Glenn <>< Garmin nüvi 2598

Speedometer calibration

Most vehicle speedometer will err on reporting faster than the actual speed. Makes sense when cops constantly pull people over for driving too fast but rarely for driving too slow.

I used to read a car magazine that reported speedometer calibration as part of the road test. It was never a matter of if the speedometer reported faster but by how much.

--
nüvi 750 & 760

my gps shows the same as thr

my gps shows the same as thr spedo on most vehicles i have driven

--
nuvi 755t

Trust the GPS

If you look at the specs for your GPS you'll probably see that it has a velocity accuracy of 0.05 meters per second which works out to 0.11 miles per hour. The accuracy is the same for regular or WAAS enabled.

Much more accurate than an analog/mechanical speedometer that can vary a few miles/hour just by the way you look at the needle.

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Gordon - Jacksonville, FL - Garmin Steetmap C550

GPS More Accurate -

I've always been under the impression the GPS was much more accurate than the speedometer.
Just yesterday, on the freeway, speedometer showed 65 and GPS shows 62. No wonder everyone was passing me - I kicked up the cruse control until the GPS said 65. End of the problem.

That is the truth

gbeyreis wrote:

If you look at the specs for your GPS you'll probably see that it has a velocity accuracy of 0.05 meters per second which works out to 0.11 miles per hour. The accuracy is the same for regular or WAAS enabled.

Much more accurate than an analog/mechanical speedometer that can vary a few miles/hour just by the way you look at the needle.

My wife always says, "You are going 85, slow down!" and I always reply, "No I am not, I am going 83" and I keep on going.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

bogus max speed readings

badliquid wrote:

I think the GPS is less reliable, and my reasoning is because when I look at what it says my max speed is, it says 143KM/H, which I haven't been up to that speed in at least 10 years, I don't go past 115KM/H.
I'm not sure why its saying that, but i ignore it, i know i'm not going that fast.

When you first turn on the gps, or when the signal is dodgy (such as when you're in an area with many loss of signal incidents), the positioning calculations are very rough. Successive inaccurate measurements can make the GPS think that it was way over there and now a second later, is way over here.... Doing the speed calculations based on those bogus positions can create the illusion of great speed, which the GPS faithfully reports just as if it were real.

In short, those impossibly high max speed readings are almost always caused by precise math being applied to imprecise measurements.

But running down the road with a normal signal, the GPS will be far more acurate than an analog speedometer.

Speedometer

I have noticed that my nuvi 350 will agree with my speedometer at higher speeds and be 2-3 MPH off at lower speeds.

Speedometer calibration

You are correct, tire size does impact your speedometer. The larger the tire, the less rotations it makes and visa versa. I had to have my whole instrument cluster replace becuase my Garmin showed I was traveling at about a 6mph difference. I always trust the GPS, I just installed the new Escort with GPS and when the radar goes off it tells me my speed using the units GPS, it is right on with the Garmin.

Live for today, tomorrow is promised to no one!

Funny Story

Years ago when I was living in New Hampshire, I had a 4wd pickup truck I fitted with oversized wheels and tires. I knew my speedometer was no longer showing me my true speed, so I had to guess at how fast I was going.

One day on my way home from work, I passed a state trooper sitting off the opposite side of the road running radar. I pulled up next to her and asked if she would be willing to clock me. She said OK, and told me to travel what I thought was 40 mph. She'd look for me and track me with her radar gun. I ran down the road about a 1/4 mile, turned around and headed back toward where she was waiting. My speedometer was probably reading around 35mph.

Again, I pulled in next to her, and she told me I was going exactly 40, so I thanked her, pulled out and continued home. Later that evening, a friend came over, and as she came into the house, said "I just noticed the registration on your truck is 6 months out of date". lol

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Funny Story

Years ago when I was living in New Hampshire, I had a 4wd pickup truck I fitted with oversized wheels and tires. I knew my speedometer was no longer showing me my true speed, so I had to guess at how fast I was going.

One day on my way home from work, I passed a state trooper sitting off the opposite side of the road running radar. I pulled up next to her and asked if she would be willing to clock me. She said OK, and told me to travel what I thought was 40 mph. She'd look for me and track me with her radar gun. I ran down the road about a 1/4 mile, turned around and headed back toward where she was waiting. My speedometer was probably reading around 35mph.

Again, I pulled in next to her, and she told me I was going exactly 40, so I thanked her, pulled out and continued home. Later that evening, a friend came over, and as she came into the house, said "I just noticed the registration on your truck is 6 months out of date". lol

--

Funny Story

Years ago when I was living in New Hampshire, I had a 4wd pickup truck I fitted with oversized wheels and tires. I knew my speedometer was no longer showing me my true speed, so I had to guess at how fast I was going.

One day on my way home from work, I passed a state trooper sitting off the opposite side of the road running radar. I pulled up next to her and asked if she would be willing to clock me. She said OK, and told me to travel what I thought was 40 mph. She'd look for me and track me with her radar gun. I ran down the road about a 1/4 mile, turned around and headed back toward where she was waiting. My speedometer was probably reading around 35mph.

Again, I pulled in next to her, and she told me I was going exactly 40, so I thanked her, pulled out and continued home. Later that evening, a friend came over, and as she came into the house, said "I just noticed the registration on your truck is 6 months out of date". lol

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Ok, I got it the first time...

wink

--
Garmin nüvi® 660, iPhone 8gb (Technology is not the solution. It's only a tool to help you achieve it.)

lol lol lol

lol lol lol

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

What's on your GPS?

We have my sister-in-law convinced that if she speeds, the police will use the GPS to ratify the ticket. Ha! Now she refuses to use one. I wonder why. 94.7 mph? Oh, I forgot, she was passing another vehicle on the freeway. What made things worse was that my brother, her husband, found it under Max Speed on the Garmin. Don't ya just luv tech?

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Garmin DriveSmart™ 65 & Traffic in Bakersfield, CA

see the "Max Speed" thread!

see the "Max Speed" thread! 94.7 isn't anything...lol. Try 500mph.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

Plane?

It had to be a plane.

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Garmin DriveSmart™ 65 & Traffic in Bakersfield, CA

Yeah, it was a plane.....or

Yeah, it was a plane.....or superman!!! LOL.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

or were you running loose

or were you running loose again in that pink costume of yours asianfire?

I couldn't help myself, you left yourself wide open for that one buddy.

Miss Poi

ROFL....dang, and i thought

ROFL....dang, and i thought that I was passing as superman.....I guess I am still the old SuperBunny

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

This was the first thing I

This was the first thing I tested and it shows the speedo a little faster.

Motorcycle

I find on my motorcycle with new tires the GPS and spedo are in sync - very close. When the tires are a little warn (8,000 miles - getting ready to be changed), the GPS reads about 5% lower then my spedo.

My Gps consistently reports

My Gps consistently reports 3 to 4 mph faster than the speedometer in my car but in my pickup they're virtually the same.

Which do I Trust?

The gps is going to be most accurate and would trust that before the speedometer.

--
Mark Ball Ground, GA

A Trick Question.

Which would you trust if your are going up or down a mountain?

Tire size

When I went to a larger tire size my repair shop connected his computer and set my cars computer so the tire size would be calibrated with my speedometer. Tire wear will change the accuracy so I always trust my GPS.

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nüvi 660 & 670 - Hello I'm a Mac :))

I guess the speedometer.

Ein wrote:

Which would you trust if your are going up or down a mountain?

Ah, interesting question..especially if it is a deep incline.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

Office

"Always trust your GPS". Did anyone see the episode of "The Office" where Michael trusted his GPS right into a lake? I thought it was funny since he could plainly see the lake in front of him. Anyway, I always assumed my speedometer would be more accurate but I guess I will have to rethink that. I have not really compared them much, so that will give me something to do on the way home tonight.

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Magellan Maestro 4250, T-Mobile G1 with Google Maps, iPaq with TomTom, and a Tapwave Zodiac with TomTom and Mapopolis

Lag Time

How much lag time is there in the GPS reading? I am thinking during acceleration the GPS reading will be slower to show current speed.

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Magellan Maestro 4250, T-Mobile G1 with Google Maps, iPaq with TomTom, and a Tapwave Zodiac with TomTom and Mapopolis

If you are going in a

If you are going in a straight line and fairly constant speed you can definitely believe the GPS. Throw in a lot of tight turns, tree cover, and speed changes and the GPS will get less reliable. Typically the speedo in the car will read conservatively. I've changed the tire sizes on my truck and it reads the other way so I have to be very careful. 70mph on the speedo now means close to 75. Yikes.

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Rusty Myers Austin, TX Garmin eTrex Vista, Zumo 550

GPS is BEST

asianfire wrote:

Sorry, I disagree and here is why. I have shot my car with radar, it said that I was going 60. My speedometer said 60 and my GPS said 60. In other words, everyone agreed that I was going 60. Now, every rental car that I have used lately said that I was going 2 to 3 mph faster then what my GPS said.

Therefore, Since my GPS was correct with my car and radar, it should be right with the rental, but it isn't so the rental is off.

Btw, I tested myself going 70, 80, and 90. At 70, my car said that I was going 70, but my GPSr and Radar gun said 68. So I trust my GPSr/radar gun more so then the car's speedometer (two over one). The same was for 80 and 90 mph as well.

OK, OK, I'm convinced!!!!!

I checked my Nuvi350 against a radar site near my home. The Radar display said 43mph. My nuvi said 43 MPH and my speedometer said 46mph

Whew! You guys are tough!

--
I have seen the future and it is now!

Awww, we aren't tuff. We

Awww, we aren't tuff. We are as soft as a baby bottom! Just wanted to make it extra clear...lol.

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Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

I read an article a while

I read an article a while back about Honda being sued because their speedometers were intentionally miscalibrated. They read about 3 mph too fast. I mentioned this to a friend of mine that works on Hondas, and he said he knew about it, and "all cars are that way".

The reason is that it causes the warranty mileage to expire quicker. Also, people tend to drive a little slower than they think they are, which gives better gas mileage figures.

The person that discovered the discrepancy did so by using a GPS system. smile

BTW, my wife has a Honda Element, and yes, the speedometer in her car reads 2-3mph higher than the GPS.

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