MPH Display vs. GPS

Best way to figure this out is drive a known distance, say 10 miles, at a constant speed, measure the time taken. So if you set your cruise to 60MPH, and you drive 10 miles, it should take you 10 minutes exactly. If the the car is going 58 MPH when your speedo says 60, it will take you 10.3 minutes, or 10 minutes, 20 seconds.

Use mile markers on the highway for accuracy of distance.

 

Generally, your GPS receiver will be more accurate than your speedometer. The speedometers in most cars tend to read slightly higher than actual speed. Of course, both measurements are subject to fluctuations caused by tire pressure, satellite signal quality, etc.

 

GPS is the most accurate, technically speaking, over distance. Most GPS units will only give the speed and direction once 4-5 satellites are in view.

Remember, the speedometer in the car is an instantaneous measurement of speed whereas the GPS speed is an average from the last, Lat and Long, PPS (Point Per Second).

If you drive _exactly_ 60mph on a flat road the GPS and the car should show the same speed.

There are variances; such as GPS antenna temperature, GPS satellites in view, wind resistance on the car, exact size of the tires on the car...
All in all, a 2 mph difference it only an error of 3%, which is nothing. If there was a difference of 7%-15%, that is something to worry about.

 

How does GPS antenna temperature affect accuracy? I have never heard of that before in all the years I've been using GPS. I do realize though that multipath errors and high PDOP can decrease accuracy. Also, the accuracy of the GPS itself is probably not much better than 30 meters, even with Selective Availability disabled. Although there are corrections applied with the street database, I'm assuming that the database is a derivative of TIGER line files - such as NAVTECH or GDT. In the geospatial sense these data sets are not considered to be large scale, and thus have inaccuracies of their own.

 

I think that two mph over the GPS is pretty good actually most of my american cars were off by more then that and my motorcycle is a bright red full fairing crotch rocket that shows 5 over. I have passed by police officers and see the laser gun pointed at me and I have never been pulled over on the motorcycle for speeding and that cushion is why.

It has saved the day more then once for me. 8)

hope this helps

 

I've checked my saturn over one of those unmanned radar carts, and its speed was dead on. Haven't found one to check the Prius since I got it.

 

I don't have the owners manual in front of me, but I distinctly remember that there is a calibration procedure for the GPS that is recommended to be done after installing new tires. Perhaps that is what you need to do to solve your problem.

 

Blue -

"How does GPS antenna temperature affect accuracy?"

Most GPS receivers use a ceramic disc as the medium between the ground plane and the antenna. The antenna is usually constructed out of tin or aluminum. With temperature, the ceramic material and the metal will change in size. This change in size changes the tuning frequency and the GPS receiver has to compensate for it. Remember, at 12GHz a .25 wave is about 6mm. Even the slightest change in antenna size will cause an un-tuned antenna.

If you read close enough in most GPS manuals, they state that it takes 5 - 10 minutes to "warm up" depending on the ambient temperature. During this warm up period, the antenna calibrates before it locks on to any satellites.

If you have a GPS receiver, try using it on the coldest day of winter after it hasn’t been used in a long time. Note the time that it takes before the first lock. Repeat the experiment in the hottest summer day. Note the time that it takes before the first lock. Then compare the times.

Unlike most electronic/computer equipment, GPS receivers are more accurate the hotter they are.

DanMan32 –

Most of those un-manned radar displays are very inaccurate. Although, it is really fun to trick them into displaying 99 or 00 depending on the firmware. Unless the radar was directly in front of you, there is a bit of error (in your favor in the case of police radar). The displays will show a speed slower than your _actual_ speed. It’s known as the cosine effect.
Here is a great image describing this…
http://copradar.com/preview/chapt4/f4d1d1.gif

 

The calibration was for map, so that dead reconning would be more accurate. Has nothing to do with speedo, and he was using a separate GPS device that provides speed as well as position, so recalibrating MFD would certainly not affect that.

 

that inaccurate? maybe maybe not. if the speedometer is 2 miles over the GPS, i would consider that normal and expected. the reason is that GPS will take a finite number of data points to determine your distance traveled. each data point assumes a straight line from one point to the other.

your speedometer will take an infinite(well not infinite but many times more) number of points to determine distance traveled. so every wiggle, every lane wandering move, every minute loss on friction or distance lost from tire compression will be taken into account.

sure, in the grand scheme of things, all those things i mentioned are nearly insignificant. but added together, i would think it might add up to... say 3% or so...give or take

ok guess i should have read the posts first because oly 57 pretty much already said what i said.

as far as radar detectors go, i dont know if you have seen the one the Lacey Police department uses, but they claim to calibrate it on a regular basis and i have checked it against my car and its dead on as long as you are at least 100 feet away. the cosine effect does matter when you get close but since they always park the thing on the street, you gotta have a pretty extreme angle before its enough to register a difference.

and the Holux GR 231 i just ordered has an average cold lock time of 45 secs but doesnt say what temperature range...so we will see

 

Thanks Dan.

I know that the calibration is for the navigation system and not for the speedometer. However, RBW111 did not specify that he was using a separate GPS device. I was under the impression that there was a discrepancy between the navigation system readout , which keeps track of distance to the destination, and his speedometer/odometer reading. That lack of synchronization can be fixed by going through the calibration procedure.

 

Also keep in mind that the velocity obtained from the GPS is through differentiation. Differentiating a noisy signal will give some error.

Also, speedometers should have a +error, not a +/- error so that the indicated speed is the same or higher as the actual, but not lower.