Garmin Speed Limit Accuracy

My guess is many people don't understand that changing sizes from the oem can cause the speedometer to be off. If you change the diameter of the tire it changes the revolutions per minute the tire rotates and this throws off the reading of your speedometer.
It's easy enough to have them recalibrated if you want to spend the $$ to do it though it cheaper to just stay within the approved tire size(s) for your car.
As far as speed limits posted on your garmin. Like mapping the most recent maps could be 6-12 months behind depending on the area in question so given speed limits can change anytime I wouldn't trust the garmin to much.

I've resolved to try reporting speed limit errors to Navteq, using the facility they provide at http://mapreporter.navteq.com/. If you use the fancy newer interface, you can navigate right down to a segment of road, see the current limits (one for each direction), and make a proposal for change.

The trouble is that for the last couple of days, the existing limits show kph values though the box bears the label mph. I don't know which unit system any correction I submit would be valued in.

Does it look this way to others? Or may I have botched a configuration option or have a browser they don't like? (my first try with Chrome hung up on some Java problem, so I am using Firefox at the moment).

I emailed to the support address on this problem--possibly after the holiday they will address a site problem, or reply to me.

I would believe the GPS. I have a Subaru Forester and it is exact with the GPS. My other car is about 2 to 3 miles slower than the GPS.

In my experience the listed speed limits are very reliable. The only speeds I've found that were incorrect were limits that were actually changed in the past couple months.

I wish there was a way to have the GPS alert me when I was exceeding the speed limit.

You might try looking at the speedometer....that usually works great...LOL

TRUST THE GARMIN

My Garmin is very close to the speedometer reading and I finally got a warning about speed cameras. My red light warnings have been beeping for awhile but I wondered about the speed cameras in Washington DC, suburban Maryland.

But here we have mobile speed cameras on vehicles so you have to be on your toes.

It took four weeks for my first answer to come--and it just said they had checked and their "database" was OK, giving a couple of examples on a road I had mentioned. I replied, pointing out that my comment was not about the database, but about the way speed limits were displayed in the MapReporter web site, and attaching screen captures and providing location links.

This got a much quicker (2 days), and positive, response, saying in part "this has been brought to the tech lead’s attention and he is going to fix this in the next production release, probably within the next couple of weeks. As this user states, currently the label for the field is saying MPH but the value is in metric, hence the discrepancy."

So, a bit slow, but with persistence, a useful response is promised.

I've noticed my Garmin usually runs about 2-4 miles faster than my speedometer. I just accepted it as to prevent me from speeding and the fact that it's tracking my speed by satalite.

I think you are saying that your car shows 50 but GPS shows 52-54.

If your speedometer is 4 MPH slower than the speed shown on the GPS I'd suggest you get your car's speedo calibrated.

It is too hard (dangerous) to drive to the speed on the GPS screen.

That much of an error means you are going to get ticketed for speeding when you thought you were only a couple of MPH over the limit.

Automobile speedometers are notoriously inaccurace. The manufacturers build in a 2-4 mph error on the high side to "help you not speed."

The accuracy markings of particular roads on the Garmin GPS's can be off because of errors in coding particular roads & changes to the speed limits by the county of particular roads. Either circumstance will get you a ticket in the wrong circumstance, unless the local rules allow for 10 mph over the speed limit before issuing a ticket by police. Some speed cameras have NO mercy. I assume that it's not accidental.

Fred

GPS speed is more accurate than speedometer or even radar. The Federal Aviation Administration is phasing out their radar for tracking aircraft while under their contol because GPS tracking is more accurate and reliable. Obviously, this will take a while to do, but it will happen.

I live in the Houston area and agree with eniku40's comments. Moreover, it's not just the south of the Houston metro area that the Garmin speed limits can be way off, it's all over the area. Garmin Nuvi saying 50 when the speed limit was 35 or 40 has happened to me more than once. And it can happen both ways, e.g. the real speed limit is 65 and the Nuvi is telling me 55. My rough estimate would be (in my personal experience) it's accurate about 75% of the time locally. Not great in my book. Navteq could do alot better. That said, I have taken my nuvi 780 to Virginia several times when visiting a friend, and there my experience was Nuvi speed limits were maybe 90-95% accurate. Seems to be potluck depending where you are - presumably dependent to some degree on the Navteq team covering your region of interest.

The only major errors I found coming down from Manitoba and return, (I-29, I-35) was along the very highly patrolled highway (#100) leading to South Padre Island. Several spots are posted 15 MPH lower than what the GPS shows- particularly around Los Fresnos.

Next year if they haven't been corrected I'm going to make a point of getting the co-ordinates for the speed limit signs and reporting to Navteq.

I have much less of a problem when the GPS is reporting a lower speed limit than posted but the other way can cost you $$$.

As of now (and not when I checked two days ago), my trial road segments display speed limits in mph, not the former kph.

So rather than just whining about locations in error, how about reporting here that you have turned in reports so that we may hope they will be corrected within a year or so?

How do I change the Speed Limit icons on my W255 so I can replace them with the ones here... for example.. (http://www.poi-factory.com/node/5649)

It can't be done as they are part of the map data.

It seems pretty accurite my car is off, 2-3 miles per mile faster than gps.

You bet. I trust the GPS better than my speedometer also.

I find that the Garmin posted speed is very accurate and changes with the sign also. At least where I have watched it.

Once I had 3 GPS devices connected while traveling and check the speed reading against the speedo on the truck-- all three read the exact same speed. I am satisfied that the GPS shows the correct driving speed.

I also think the gps shows more accurate speed.Set my speed control based on it and no tickets after driving 800 miles.

My 855 reads about 1 mph faster than my car's reading. I think that is rather good agreement for two different devices using completing different means for the measurement.

In the month and a half I have had my 765T, the one consistent thing I have noticed with regards to errors in the speed limit information is they are too high. Generally this is due to many of the areas I travel having lowered the limits; 1) two areas went from 55 to 50 2) one area went from 55 to 40 3) one area went from 40 to 30. I have yet to go to an area where the GPS says the limit is lower than it actually is. Basically, IMHO, the various jurisdictions are lowering the limits in an attempt to raise revenue.

Garmin's speed limits are not quite up to date, but as a reference they are ok.

I recently upgraded from a streetpilot to a nuvi. When I had the streetpilot I would always think the GPS is able to to calculate my arrival time, so it must know the road's speed limit. It would be great if it would share that with me, but assumed it was kept hidden for liability reasons (as the OP encountered). However, I was thrilled the first time I saw it displayed on the nuvi.

Hello,
I think my Garmin's speed is more accurate then car's odometer. The only thing is when you stop sometimes gps still shows that you are going 5-6 km/h for a split of second. And in reverse when you are doing 20 already gps might just show 10-15. It's because of satelite signal delay. It's not significant as it lasts for no more then 1 second. As soon as you reach your cruise speed it's very accurate. Good luck!

I've noticed in 3 different vehicles with the same nuvi 265wt a variance of about 2-3 mph in all three vehicles.

Just got back from a trip into the US and the Garmin speed limit feature kicked in for the first time on my 265wt. It was really helpful, it did the miles / kilometres per hour conversion flawlessly, very handy. It seemed quite accurate, changing up or down along with the speed signs posted alongside the road.

I don't expect a construction zone's speed limit to be reflected on the gps. Really, how could they keep up with that. I do, however, expect a highway that raised it's speed limit from 60 to 70 a year ago to be updated but...