Which do I trust? (speedometer or GPS)

They all use an audio tone and the Doppler effect to measure the rate of change. If the transmitted tone is not on frequency, the received tone shift would be off.

I am thinking about all those years I did not have a GPS and my wife leaning over to see that I was speeding (5mph over) and telling me to slow down... now the GPS has vindicated me as being correct in the first place.

My garmin is on the money for the most part with both.

My GPS is always 2 mph less then my speedometer. Not sure which one is right but I would think the speedometer is the winner on this. I would thhink there are some delays with my actual speed and what the GPS is picking up.

The GPS is doing is calculating distance change/time. Even if you are currently have a 15 ft inaccuracy it does not matter, all the math is doing is looking at change... now if the inaccuracy changed during the sampling that will effect the results but if the GPS is doing this once per second then you will not notice.

This is a great coincedence this quandry came up at this time. Over a period of a month, recently, I checked my speed with the second hand on my watch, using mile markers. Did it several times in fact, with all 3 vehicles. Each and every time, my speed was almost exactly with what my GPS said it was. My speedometer seems to be exactly with my GPS in only one vehicle. I know that my math was correct, so I think that the slight deviation in speed was probably my own judgement as to exactly when I was passing a mile marker, and the deviation in my own cruise control. I did the experiment out of bordom and curiosity. So, near as I could tell, my GPS speed calc seemed to be exactly right on.

Moving to a larger tire will make the speedo read slower than you re actually going. A larger wheel will have to spin less times than the stock tire, so the speedo would have to be recalibrated.

You are right in that radar uses a tone's frequency to determine speed. Where you error is that the tone is the result of mixing the transmitted RF signal with the RF signal reflected from the moving car, not modulation of the transmitted signal. A car moving toward the radar gun will shift the reflected signal's frequency up while a car moving away will shift the frequency down. The amount of that shift is a function of the speed of the car, the Doppler shift. If you mix the received signal with the transmitted signal, the result will be an audio tone the frequency of which will indicate the speed of the car. It makes no difference what the frequency of the transmitted signal is, the shift in frequency is all that the radar is interested in.

If you measure that audio tone with a digital counter, then no calibration is needed as long as your time base for the counter is reasonably accurate. That would be set at the factory and not be field adjustable.

The first radar units used a type of discriminator detector that produced a voltage that was dependent on the frequency of the tone. The readout was a analog meter. The meter would swing one way if the car was approaching and the other if it was going away. The discriminator needed to be calibrated quite often and that was done using a tuning fork.

The signal the radar units transmit is not modulated with a tone. It is a CW or continuous wave signal.

I hope this helps.

Jack j

I used to have the same speed on both, but then I changed my tires to new ones and the GPS says I'm going a little faster. I'm guessing the car counts rotations perhaps and the GPS is going by distance. Seems to make sense as my tires are slights larger now than they were before.

If you check the tires on tirerack you will notice the number of revolutions per tire. In the case of my Prius the closest I could get to the factory installed tires was a 0.05% acurracy

On both my GM cars the speedometers are accurate, at about 70 mph, within 2%, based on 10 minute timing runs between highway markers at various places.

My nuvi 855 is supposed to be accurate within 15 feet. I'm not sure what that means, since if I look at the tracks left on the map, especially when parked, I can be upto 100 feet out. I surmise the specs mean within 15 feet 95% of the time. My unit seems to update about every second or so, this means lots of seconds that it can be outside the 15 feet.
Of course, we all know that the gps is not at its best when stationary, so assume the 15 feet is good at 100 feet/sec, or close to 70 mph.

- If the inaccuracy is constant, there will be no effect on the speed calculated.
- in the worst case if the inaccuracy is + 15 ft and then - 15 ft. the speed will be 30% wrong.
- if the inaccuracy is +1 foot and then - 1 foot, the speed will be 2% wrong.

In my case the gps continually flicks between 68 and 70 mph, when the speedometer indicates 70 mph. Which is correct? Both are within the limits I expect.

At lower speeds such as 50 ft/sec and using the same logic, the gps will now be about 4% wrong. I haven't been able to calibrate my speedometers at these speeds so I don't know which to believe.

The 15' accuracy is the best you can get and it varies with signal strength. You can see the accuracy on the nuvi by pressing the satellite strength bars with your finger and holding it for a few seconds. Then you get one of those hidden screens with the relative positions of the satellites and the accuracy.

With my nuvi 769 I have seen variations of anywhere from 8' to over 50'.

Typically my 855 is within 3 to 5 metres, hence my use of 15 feet. I have seen as bad as 10 metres though.

It's not the same amount at all speeds, but the variance increases as my speed does. By the time I get to 70MPH on the GPS, the speedo is reading about 5MPH less. I've thought about getting it fixed on several instances, but it's not enough of an issue to worry about.

Even the same thing is seen in BMW's...they have a 3-4% speed error margin.

When it's showing 30 on the gauge, the car is acutally going 26-27 mph...same at 70...is really doing 66.
If you bring it in, they will say it's within limits.

When my tires were really worn, it would be on the 4mph off side, with new tires, it's on the 3mph off side.

I've noticed my Nuvi 660 is always 2-3 mph faster than the speedometer in what ever I have driven(3 different vehicles)

Mu GPS is always 2 - 3 mph slower than my speedometer. I think this would be a useful lawsuit.

So is mine. I trust the GPS over the speedometer.

Your max speed can be outrageous.. Whenever you move before your GPSr is locked on the sats it will calculate that move as speed..

The larger the distance, the faster the top speed.. When I was driving truck, and my GPS was always on my max speed stayed withing 1-2MPh of what my truck would actually do.. Governed to 68MPh max

When I was in the truck I drove my speed by the GPS, and as others have posted I was never off on the little "radar trailers" and I've never had a speeding ticket.. Even when I drive around with my old Garmin GPS-38...

Off by more then 1 (or 2) mph why not have the dealer check your car (assuming it's under warranty)? May need to be adjusted if you have different tires.

The question is which do you trust. The answer is they both should be accurate and both should give you the same (or virtually the same) speed.

my nuvi always agree with my vehicle when I set to cruise control on flat and straight road.
If one is wrong, the other is wrong, too.

Unless your spedometer is broke, use the car's and not the gps.

speedometer read from transmission not tires to be more accurate, the GPS read from 3 different SAT. is more accurate.

Although not much, speedometers are subject to wearing out and becoming inaccurate over time. GPS's reading satellites are not.

How about modern cars? Speedometers are just sensors and digital displays. No mechanical parts.

They have sensors in the transmission or the ABS systems.

Just the scientist in me, but I still think that triangulation with 3 or 4 satellites is more accurate, while the speedometer is still reading a mechanical action.

I am traveling 60 mph on a flat road. Then I encounter a 6 percent grade. I maintain my 60 mph on my speedometer. However 60*6 degrees cosine = 59.67 mph on a flat map. what will my GPS read for speed?

Depends on whether or not you define speed as horizontal velocity or angular momentum.

The speedometer will measure your motion in relationship to the road, the GPS will measure your progress through space.

Let me ask my daughter or son-in-law, who have Masters in Engineering degrees.

You're making my head hurt!.....

I find this interesting that this thread has gone on for so long. There are many reasons a speedometer could be inacurate. the GPS is measuring the change in location for a period of time and calculating the speed. if your are on flat ground there is no doubt the gps is the most accuate.

I find this discussion interesting. Go on...

i had a doozey going for a while, while going down the highway i was going app 60 mph on my speedometer
the garmin said i was doing 16 mph and when i got off onto a street it corrected itself to 26 mph so much for that lol, if i wasnt getting a satelite why did i get any speed indication at all? i love my garmin but some time i think it likes to screw around with me lol like a bad child.

My thanks to flanga. Now I know why I get such a reported high max speed when I trust the gps to accurately report my current speed. Last year, I got a ticket when I wasn't aware the speed limit had dropped by 10 mph. The radar gun readout showed exactly the speed I had set with the cruise control using the gps as reference

You were in a worm hole.

Long in a linear sort of speedometer way, or long in a three-dimensional GPS sort of way?

Yep, that IS the question!

Surely this is an old thread but still interesting.

Below is a link indicating the GPS is more accurate.
http://gpssystems.net/accurate-gpsbased-speedometer/

Businesses use Fleet Management software to ascertain if their drivers are obeying the law with information provided bu the GPS

My own vehicle displays 2 MPH faster than the GPS.
I have read in other articles, in addition to being mentioned in the link, car manufacturers intentionally do this to err on the side of safety.

Now my head hurts also.
http://www.golfwrx.com/forums/topic/162073-gps-how-does-it-m...

think about it - u can get location to within 10-20 ft from a device that is 11,000 miles above u and u r questioning the speed accuracy?
DUH, now altitude is something else and might not be satisfactory for landing a plane, but for telling me when to but on O2 it is fine.

I never thought to compare the two. Now I know...thanks. cz

by what the cop says I was going!

GPS for sure

I drove from Ohio to HHI SC today and the speedometer and GPS were in step; the same holds true for my other vehicle. I would look to the GPS first as it is the most consistent. Tire pressure, size etc all can impact the true speed of the vehicle.

which ever one is calibrated

The GPS is constantly calibrated...

As a point of interest... Garmin claims a velocity accuracy 0.1 meters per second. (This is listed in the 'specifications' section of most owner's manuals.. But not all owner's manuals). Converted to Imperial measure, that's an accuracy of about 1/10 of a MPH.

BUT note that this is with a good satellite fix at a constant speed on level ground.

If they are using the word 'velocity' correctly, it has also includes inclined roads, not just level ground.

My guess is that speed calculation have been affected by error on your location. In city, GPS accuracy can be affected by signal rebound on wall , especially if signal is week + few sats are in sight.

In optimal condition, which in my view are: Highway, no High building nearby, strong enough gps signal , multiple satellite reception (over 4), AND constant speed (I think gps will be slower to react on speed change, since it's base on multiple location / time calculation) and probably on a road with constant altitude , I do believe that GPS is by far more accurate than speedometer.

Even if all those conditions are not present, I think that GPS is still more precise than speedometer most of the time!

Disagree--standard GPS practice is to use Doppler heavily for speed calculation.

Agree--despite plenty of sources of short-term error (multipath is important, for example), any sustained difference seems almost certainly the fault of the vehicle speedometer.

Why is it that some threads that deserve to die a nice peaceful death don't?