is the mph data accurate?

I think the gps is right. My nuvi 360 always shows 1 mph faster than the speedometer in our car.

I can think of lots of reasons that the car speedometer would be off, but not one good reason that the gps would be consistently wrong.

Yeah, the GPS unit is much more accurate than the car speedometer.

That's one way to shorten a warranty period. I would have never thought of that.

Honda had a lawsuit against them for having the speedometers set fast.

Your indicated speed is determined by measuring the Doppler shift of the satellites and is very accurate.

The owner's manual of the Nuvi 660 states the speed is accurate to .05 meters per second.... That's about 1/10th of a mile per hour. In reality, you can expect around .5 MPH accuracy.

The speed shown isn't instantaneous, it's averaged over about a second or two. Unless you're accelerating or braking rapidly, this averaging isn't that noticeable. At a steady cruise, you should be bang on.

FWIW, I have three vehicles and the speedometers all read two or three MPH higher than my GPS.

When I’m on the Highway my satellite reception jumps between 12 and 25 feet depending where I am on the highway here in NYC. So the better the reception the closer it is to what my cars speed reading is, that’s what I’m noticing.

So am I correct that the gps is off because of the reception and that the better reception will cause the gps to come close to your cars reading of speed? Then that would mean your cars speed reading is correct and the gps reading is false?

You may be interested in this thread:
http://www.poi-factory.com/node/6695

RT

Subject has been covered in GREAT detail also at:

http://www.poi-factory.com/node/9247

I guess I would stick with the speedometer due to the gps signals being off. What I mean is that the better the gps signal, the closer it compares to the speed I’m doing. I usually drive ten miles over the speed limit which won’t get me a ticket. I’d hate to get a ticket due to what my gps is showing.

I agree....I continually use my GPS in rental vehicles...the GPS is about 3-5 MPH lower than the vehicle speedometer in all vehicles:)

mime has speedometer and gpsr agreed in cars I know of.

There is a class action lawsuit against GM for defective speedometers in trucks. My tahoe speedo reads 75 when parked. I depend solely on my GPSr until the lawsuit is settled and GM replaces my speedo.

I trust the GPS more than the speedometer ... Out in the open that is.

the GPS is more accurate than the car spedo/odo. the car speedo/odo is dependent on the size of the tires. manufacturers typically put the smallest on when they calibrate the speedo and odo. this is to prevent people from putting undersized tires and claiming less mileage.

GPS is more accurate. When I had a pickup truck, it had bigger tires that threw the speedo off. So to recalibrate the speedo, we use a gps.

Just something interesting. Our Toyota Camry's computerize 'Fuel mileage' indicater showed 42 mpg with long-distance driving 65 mph on the interstate.

Even with a 4-cylinder, 42 mpg??? So I checked it the old fashion way ... actual 34 mpg. When I had it at the Dealer, I asked him to adjust it for better accurracy. "No can do .. no calibration available. Besides, this is only an estimate." Estimate?? Heck, I can throw a dart at a dart-board and get a better 'estimate' than that. Hmmm, I wonder why it isn't off in the other direction!!

RT

I have also experience this.

come up with just about the same mph's than my speedometer does. As you said, there are many factors involved, tire size etc...

Those here (such as me) that have VW cars and the VCDS software and cable (allows user to see ALL computer info in the car) know that the car speedometer usually reads high but the odometer is almost always dead on. The speedometer reads high so the auto manufacturer cannot be held liable for speeding tickets. There will be errors in the speedometer, but they will almost always be on the high side (speedometer reads faster than the car is actually going). If I plug the VCDS software into the computer on my car the speed information given by the car's computer is dead on with the GPS, but the speedo has a built in error once the speed goes above ~45mph, reads higher. If you are worried about your odometer why not check that with the GPS, or bettter yet use the mile markers found on the side of most major highways.

Has anyone seen these on highways? Basically, there would be a two or three-mile stretch that has its own mileposts, and somehow they're supposed to be used to test the accuracy of the speedometer.

I tested my 660 many times with the cruise set, and it was dead on every time. I do have a new set of tires after the purchase, so tire wear could be a factor. I am not sure if I tried it at below 50mph though.

I was a navigation engineer back in 1980, worked 3-way satellite systems, and even with just the 3 sats, the pinpoint positioning was supreme, at higher frequencies. Now we have so many sats in the sky, I'd believe the GPS automatically for speed.

In my truck I trust my GPS because the factory one no longer works but I have noticed in my van that the Max speed feature sometimes reads alot higher then I ever saw on the speedo (gps or factory). So if I ever get pulled over I will not use it to try to prove to the Cop how fast I was going, if fact I will probably reset it before I stop.

In visiting with some KDOT employees in Kansas - they say that mile markers are only reference points, and that they are no longer a mile apart. This is due to the fact that as they change roads for new bridges or curves they do not want to move all the mile markers for the state. A better test (at least in KS) is to check along the crossroads, as they are always a mile or half-mile apart. In my cars even if the speedo was off, the odometer was always correct.

Daniel

I drive a wrangler jeep and the speedometer is off by 3 miles. I had it check by radar. i use the speed on my 660 and it show the same 3 miles difference for the speedomete. It is righe on with the one in the motorhome. I have noticed that the odometer is very accuret and I check it on the interstates when they have a 5 mile test section. I don't think the speedodometer will make a difference in the odomer as a mile is a mile no matter how fast you get there.

I set my dad's Toyota Sienna LTD, with cruise speed set to 65 mph. It agrees with GPSr reported speed. I can't speak for owners of other makes/model though.
But, this may be a test worth to try. Please conduct it carefully. Do it on flat and straight road under speed limit, when traffic is not heavy.

My VW Jetta has an odd problem with the speedometer. Once I hit 80+ mph and reach 4K RPM, the speedometer stops working correctly. The needle bounces between 70 and 90 mph...so I have to use my 760's speed data regularly.

Going past those police speed detector trailers that show you your measured speed, I've found that my Jetta's speedometer reads 2-3 mph faster than what I'm really doing, however, my GPS always gives me the same number as the police trailers.

The speedometer on My ford F150 was 3mph off at 70 mph while my wife's Honda is 1/2 mph high at 70 although I got a new set of tires a few years ago and made the ford speedometer only two mph faster. They were supposed to be the same size but must have been a wee bit bigger. I thought that the wear might make a difference but it doesn't seem to.

I drove my wife's Toyota Camry on a trip last year and checked the odometer in Utah over 100 miles of mile markers- It was excatly 100 miles- I was surprised it was that accurate. I did not have a GPS, so I did not check the speed part.

This info totally agrees with what I found on my car forum. So that means the needle always reads high, but the odo is correct. I suppose some of the larger states such as CA would file a class action on the warranty issue that would arise if the odo reached respective numbers sooner than actual.

Ok, now to my confession, I wanted to have my max speed read triple digits, so I cruised at what I thought was 104 mph. To my disappointment, the nuvi 660 read only 98, and I've not been in another situation where I could drive that fast since.

To me, that's a huge discrepancy. That means at 155, the actual may only be 146. I've seen many a person post on the web a shot of their speedo nearly pinned on the autobahn when they picked up their cars in Germany, seems like they could be dealing with 9 mph less than stated on the speedo.

Trying to find out if this is lawful to bring GPSr aboard a commercial aircraft. This allows to see if it can report three digit speed reading. My understanding is, not all airlines allow the use of GPSr on board.

Have anyone ever done this before? What is the GPSr reading compared with reported speed from cockpit?

You can do this without a computer, by using the A/C display in models with electronic climate control (i.e Climatronic). By pressing a combination of buttons, you can enter a 'diagnostic mode', that shows the real-time data for the sensors. Current speed is displayed on one of the channels (17 on mine) and it is the 'real' speed that the ECU utilizes, as opposed as the one displayed at the dash.

This reading matches the GPS speed, which is 2-4 mph slower than what the dashboard indicates.

This works in VWs as well as other VG brands (Audi, Bentley, Lamborghini, etc.)

Krieger

I have a Mio c310x and I found the MPH info to be somewhat accurate

I would definately say the GPS is more accurate than the speedo.

Also, I don't think the speedo and the odometer are related... so although the speed is usually overestimated, i think the milage will be pretty close to dead-on (unless tire sizes have changed)

I did test the 660 at 30mph along with another check at 55mph, in cruise control. The 3000GT spedo and the garmin was dead on the money. I had a tad of uphill/downhill along with flat road, both the GPS and the spedo changed by +1mph downhill and +1mph uphill.

I also brought the satellites up on display for fun, and I was a bit surprised thet there was a reported 12-15' accuracy limit. Back in the navigation work in 1980, 3 satellite tracking gave no more than 1 meter of error. We did set/calibrate the unit at established markers(stakes in the ground) prior to the job, but yet I would have thought that the added sats the GPS reporter would have triangulated even better. Maybe it is simply the established cal on the receiver, which would explain that I saw 15' and 12'...3' vasilation, darn close to a meter. The map overlay might have benchmarked the 12 or 15 foot skew, who knows?. It would be cool(a bit anal as well) to be able to adjust the garmin at an established pin-in-the-ground of location. Being a bit of a techno-nerd, I know.

When the job was marine oil exploration, the customer would have been really pissed after dropping a rig if the Global Positioning was not pin point. Satellite nav systems were just coming on line back in 1980, most of the work was done with a lower frequency ARGO system and established base stations, three for accuracy, low frequency for the range out to 200 miles offshore. My point is that with today's GPS capabilities, exactness of location and speed tracking HAS to be the best, especially compared to a car's speedometer SV

http://www.poi-factory.com/node/2590 - thorough discussion of the questions you ask...

On my acura, the speedometer and the streetpilot 550(which is what i have) are dead on as well. In fact i have a garmin forerunner which i use 4 running and that was in sync with the car's speedometer as well.

Thanks, KCH. I went to that thread, and left couple of posts. Very interesting.

There is no law against electronics in an airplane. It all depends on the individual airline's rules. And yes, your GPS will agree with what the pilot tells you.

I asked a police friend to check out my Garmin c340 in his police car which has the speedometer calibrated at least twice a month and he said that it was within 2 mph. That is not bad, wouldn't you say.

Within 2 MPH is terrible.. Garmin claims a speed accuracy of .05 meters per second for the C340 which is about 0.1 MPH (Almost all the modern Garmin products have this accuracy).

http://www8.garmin.com/products/sp340/spec.html