Which do I trust? (speedometer or GPS)

I purchased a second GPS about a month ago, and both my Garmin and TomTom 720 agree with each other. One nice thing about the TomTom is it knows what the speed limit is on a stretch of road, and if you start to exceed it the bar at the top turns red. I still prefer my Nuvi, but the larger screen on the 720 is easier on the eyes. Even booting as a 940 (mine's a dual boot), the TomTom GUI is still inferior to Nuvi.

My car speedometer is always in sync with GPS.

I took some code (available online) and modified and uploaded to GPS. This code took lat long and saved in a text file. Then I drove my car at constant 75 (cruise) on flat road for 5 minutes and cruised at 45 for 5 minutes.

My speedometer and GPS agreed whole time.

Then I downloaded the dats stored and run it over script which took this latitude and longitude, and used google scripts to calculate speed. The speed ranged from 74.9 - 75.1 (for 75 mph stretch) and 44.9 - 45.1 (for 45 mph) strech.

So I believe GPS is very accurate.

The discripancy may arise due to following reasons:

1. Speedometer not caliberated very well.

2. The tires put in vehicle is slightly smaller/larger than the tire size used in calibration. The difference can be small (1/10th of an inch). Even a wear/and tear or brand new tires can make difference.

3. Going on inclination (uphill/downhill) . The GPS algorithm do not take into account the height difference in speed caluclation. The GPS are not very good at altitude measuring. The difference can be in 10s of meters. But since GPS usually take one or more readings per second, the inclination has to be rather steep.

My two cents.

I was always told as a kid -----

DON'T TRUST IF THEY ARE OVER 30 !!!!!!

I guess lower speeds are OK.

my garmin c320 show different speed with speedometer

i think right or wrong ??

Actually,the GPS calculates a average speed between the coordinates as it passes through them, the more coordinates it calculates within a time the more accurate the reading is for the speed.

So if you have a high quality GPS receiver making (calculating) more positions over a given time period them your speed accuracy increases.

Beware however - I believe some units actually do a rolling average so any short term change of speed will not get a true indication until the new speed has bee stable over the time period required....

My 360 and my new f150 is within .5 miles of the roadside radar signs. and is right on with my wind-star handy cap van but about 3 miles off on the wife's ranger since we installed the new tires. I have noticed that if I let the tires on the van get a little low on pressure the speed gets off about 1 mile at 60

gps is more accurate than your car speedometer

Garmin states that at a steady speed, their units are accurate to within .05 meters per second, (which is 1/10 of a mile per hour). All the modern GPS units are in this range.

The speed accuracy is usually written in the specification section at the back of most owner's manuals.

Even the size of the tires changes the accuracy of the speedometer. I would go with the GPS for speed

All because of the following:

I generally keep my top speed within 60-75 MPH, depending on how fast the traffic moves on the highway, my GPS has clocked me at max speed of 140+.

When stopped, often the GPS reports I am moving at a speed of 5-10 MPH.

These two should be enough of an indication that the GPS isn't as accrue as the speedometer.

Actually have a cop as a friend and my Garmin was closerby two mph. Could be the winter tites though.

i trust my gps

I have compared the GPS to the Speedo on 3 cars + 1 motorcycle. The GPS is correct when running through the 'your speed is' signs. The cars all display high (speedo says 60, actual = 57), motorcycle same, but off by 5 MPH.

Tim

You have a faulty unit.

If it's a Garmin and still under warranty, then send it back. The error you are experiencing is FAR greater than the steady speed accuracy of 1/10 MPH that Garmin claims.

I doubt it ...

All of this calculation has to do with the speed and accuracy of your satellite reception. This factor alone introduces an errors of +/- 50 to 100 feet per every time a speed calculation is being done, depending on your location.

IMHO it is very unlikely I am the only one that have been experiencing this with their unit.

jc

mine are very close, if I am cruising I see no differance

If I recall correctly you're in New York right? Maybe the concrete canyons have something to do with it?

Both the GPS and the speedometer is exposed to factors influencing the "true" speed:
http://www-group.slac.stanford.edu/met/GeneralMet/AvsP.html
I would guess that "precison" is always better on the GPS, compared to the speedometer. A better precison do you obtain by calibration.
You can easily read the accuracy on your GPS, it varies, as you know.
We dont know too much about the accuracy on the speedometer, it is probably very different between speedometers. When I was a kid, the speedometer "needle" jumped up and down, bad accuracy.
You can't calibrate a bad accuracy.
A rifle has in general a high accuracy compared to a shot gun, BUT both could have a very high degree of precision!
In general terms: We increase the precision on our rifle by adjusting the fore sight! We buy a new rifle in order to increase accuracy (or try to always have a steady hand when hunting!)

Of course they do.

There is not one day that I drive in the city and my GPS "physically" indicates I am two blocks away (50-100 ft).

I guess the point I am trying to make is the speed reported by the GPS is only as good its reception. We can assume that in open highways the data will be better then within city limits. In addition, we also need to consider other factors, such as weather condition, before we swear by the GPS over the speedometer.

I believe the GPS is correct. If your car is Japanese or Korean, I'll bet that the speedometers are reading high. Honda recently settled a class action suit because their speedometers were reading about 5% high and part of the settlement was an extension on the warranties.

I have a Kia and checked the speed against the highway kilometer markers on the highway and the GPS was consistently correct and the speedometer was consistently wrong. i also had a time when I drove past one of the radar signs that displays your speed and again the GPS was right.

I had a Hyundai and it also read high. I have to wonder if they are shaving their warranties this way or worried about a lawsuit, if someone gets a ticket for going to fast.

Maybe a good precision, but bad accuracy!!

This is an indication that you have a really poor lock on the satellites, hence the movement while stopped. You should make sure the unit has a clear look at the sky and your windsheild doesn't have a metallic coating.

The top speed is because you were either moving, or had moved some distance from where it was cut off when you started it. It considered the difference in location, movement, and set the speed accordingly.

I never use my gps speedometer. Maybe thats because on my garmin 350 the speed is not shown on the main screen. I find this discussion interesting though. I am going to check my readings on my next trip. I do know tire pressure can effect the accuracy of the car's speedometer so it could be a good tool to alert me to a possible low tire problem.

Automobile speedometers are not calibrated as closely as GPS, assuming you have a solid multisatellite signal.

Tire circumference changes as tires wear, the "paranoid" me noticed that my car reads 7% high - warranty to end 7% sooner that way. Also highway mileage markers can be off by quite a bit (try checking every mile for 5 or 6 miles and see how many feet you are off from mile to mile.)

However, for total mileage, be careful using the GPS, especially if you stop. Random signal errors due to propogation, processing issues and the like can cause the GPS to think it is moving when it isn't.

I have a new Hyundai and it is reading the speed high. (Speedometer 64/ GPS 60). Won't get a speeding ticket that way but will certainly use up the mileage warranty significantly quicker. May look into class action for Hyundai. I know I am not alone.

Agreed, I would trust the GPS. Having said that, I was surprised finding out how accurate my speedometer is. Between 30 and 75 mph, the speedometer reading on my Toyota Echo is right on the mark (at least to the accuracy you can do the reading of the dial).

Mine seems to be off by only 1 maye 2 mph. I just put new tires on so maybe as they wear that will change???

My GPS and my speedometer agree completely at speeds under 80 mph. I must be fortunate since my front tires are newer than the rear ones and my truck is 5 years old. Lucky me.

The GPS will be much more accurate in most situations. A speedometer can be affected by something as simple as tire size which can be affected by pressure (although to a small degree)

The instantaneous rotation speed of a speedometer cable varies over a short period of time. To keep the speedometer needle from jumping and vibrating, speedometer needle is damped. This causes a reading lag similar to that of a typical 1 sample per second GPSr. Instantaneous speed is of little concern to a policeman, since the police radar can read neither instantaneous speed nor instantaneous position. If you take you track log to court, it should stand up to any evidence a policeman could present. If your track log say you were speeding, pay the fine, take the class, you have no chance in court.

Use the total distance to check your odometer as well. You might be VERY surprised. Mine was over by 3%. Think of what that does to new car warranties.

I trust my gps.

Yeah always trust the gps!

The GPS is always bound to be the closest as change in tire wear and pressuer can have a big impact on what your speedo is telling you. I have found on teh motorcycle the GPS and Speedo are always in agreement right after putting on new tires then over time they start to drift apart - until I put on new tires again...

This has always confused me! I go by my gps, no logical reason, only that's it's faster. Maybe I'll print this out and show the next Officer of the Highway that pulls me over for speeding.

Was your puppy sick?

Actually,the GPS calculates a average speed between the coordinates as it passes through them, the more coordinates it calculates within a time the more accurate the reading is for the speed.

Actually, coordinate accuracy has nothing to do with speed accuracy when using a modern GPS.

Any current, decent, GPS measures speed primarily using the phase shifts of the carriers, not positional changes vs. time.

I don't think any car less than about 15 years old uses a speedometer cable. The speedometer is driven by the car's computer, same for the tachometer. The days of the mechanical gauge is over.

Jack j

Trust the GPS. I have a Garmin 680 and a radar detector which has a built in GPS. They both register the same speed which is always slower than my speedometer.

From the experts I've heard from they say the spedometer in the car is pretty inacurate to the point they put in a seperate one that's slave to the wheel. I heard a case where they were debating on whether or not to allow GPS data to show a speed camera was off

The speedometer of a car is set, not to the tires, but to the transmission. It is the revolving of the transmission (in simple terms) that is reflected in the speedometer. The car speedometer is calibrated to reflect the factory spec tires when they are reasonably new.

As the tires change from the original spec, in size (14, 15, 16, 17 inch), air pressure, profile, wear, etc., the speed that the vehicle actually moves will change in relation to the actual transmission speed. Smaller circumference tires, due to all of the above changes, will spin faster to cover the same distance than larger circumference tires. Smaller tires = faster indicated speed.

Trust the GPS after it is stabilized. On an open, straight road with good reception, it is extremely accurate. In a city with big buildings and poor reception, it can be a bit dicier.

Will walls of glass reflect signals from satellites? (lol)

looks like, tire size can create an error of 2-3 miles an hour at speeds of 20 - 60 m/hr. 1-10% in error. Wow that will result in 1 - 10% error in total milage of the car. That changes games for many, e.e. insurance (they charge you extra if you go more than 6000 miles / yr), also car dealer lease and the list goes on.

I will want to have tires of exacr right size, so that I do not loose on these (insurance etc).

I have been running oversized tires on my truck since I bought it. And, I have checked the odometer a number of times over the course of the life of the vehicle. There is NO question that it has reduced the speed indicated on the speedomter and the odometer. In fact, I have confirmed multiple times that it is showing about 2 1/2 percent less than what is actually happening based on the mile markers along the road and doing this over a long distance - i.e. over a hundred miles. Use the GPS for accurate speed.

My vote for accuracy is with the GPS.

I would go with the gps. Yes, it has some distance, but for the speed your going it should be more correct. A car has alot of moving parts that could change your speed. Tire size, air pressure, cables ...ect.. over time things will also wear down.

I trust my Garmin GPS.

A few years ago I was down in Washington DC, with my old Quest unit. I pulled up to a red light, glanced down, and noticed the Quest showed the car was still moving - it actually showed me driving at over 20mph! After about 15 seconds, I got a "lost satellite signal" message. That's when I realized I was on the street alongside the White House. I don't know what they had going on there, but the GPS sure didn't like it!

In both my cars, there is insignificant variation between the GPS and the speedometer.

It's not calibrating the speedometer but changing a gear in the trany that changes the reading.