| Author | Message | ||
| Austin Scott Davis (Zimtok)
Registered Member Username: Zimtok Post Number: 277 Registered: 9-2006 Posted From: 216.37.73.226 Rating: N/A |
How accurate is your GPS device when it comes to your speed? I have a Magellan 1400 and the MPH seems to be off. I have had several people folow me in the bus and swear I'm going faster then the GPS indicates. The GPS will read 61-63mph and the speedo of other cars will be around 65-70mph. I know this is not much as all vehicles are a little different anyway. But I would expect to see just a few that were off on the low side also. Every one of the "chase" cars have reported my speed to be faster then the GPS indicates by about 5mph. Just how accurate is a GPS? | ||
| John and Barb Tesser (Bigrigger)
Registered Member Username: Bigrigger Post Number: 94 Registered: 9-2007 Posted From: 24.197.246.104  Rating: N/A |
I suppose they are all different, but my Tom Tom is almost dead on with my speedo. | ||
| Luvrbus (Luvrbus)
Registered Member Username: Luvrbus Post Number: 503 Registered: 8-2006 Posted From: 74.32.83.140 Rating: N/A |
I would think that they could be different mine will get you in 3ft of a property pin and the old one I had a few years back was 35ft | ||
| Bruce Henderson (Oonrahnjay)
Registered Member Username: Oonrahnjay Post Number: 263 Registered: 8-2004 Posted From: 72.12.39.78  Rating: N/A |
I was involved in a couple of complex GPS projects a while ago. Things are better now (the US Government has released a number of more precise algorithms in the past 6-7 years, but you need to really dig into GPS to understand it. Especially with "civilian" GPS, many times the unit figures with "logic" not entirely complete information. Especially if a supposedly accurate satellite is a little off, the unit will make an error -- it's kind of like a JPEG photo, some of the "blanks" are filled in by logic and not a precise bit code. In reality, GPS is usually quite accurate as the 3 ft from the property pin will show. The problem is that when it's not accurate it's not accurate and only a few will tell you when they're not accurate (and even if they're working on "logic", they're usually giving accurate readings anyway. If you were to walk out to the property pin once a day for a month, you'd probably find it perfect for 29 days and off once but there would be no way to tell the difference other than careful testing.) So, you can expect GPS to be accurate except when it isn't. (However, if you've done multiple tests but GPS and some other data don't agree, I'd first assume that the other data is off -- especially, speedometers, they're never right!) | ||
| Laryn Christley (Barn_owl)
Registered Member Username: Barn_owl Post Number: 508 Registered: 10-2006 Posted From: 72.66.169.73  Rating: N/A |
Car and Driver did a speedometer test a few years back and they almost consistently found that all car manufactures speedometers read slightly higher than actual. That included the likes of BMW. So your GPS might be right and everyone else wrong. Car and Driver - Speedometer Scandal! http://www.caranddriver.com/reviews/hot_lists/high_performance/features_classic_cars/speed ometer_scandal_feature | ||
| Nick Badame Refrigeration Co. (Dnick85)
Registered Member Username: Dnick85 Post Number: 246 Registered: 2-2006 Posted From: 75.196.192.37  Rating: N/A |
Hi Zimtok, GPS devices today are very accurate. In the past 3 years, US Global gps sats have become the best out there.. At our recent KVH class, we were told that our gps systems are accurate within your Pinky finger. Not 3ft which was true a few years ago. I would trust your gps device over a speedo anyday. Nick- | ||
| niles steckbauer (Niles500)
Registered Member Username: Niles500 Post Number: 846 Registered: 11-2004 Posted From: 71.180.67.29 Rating: N/A |
Keep in mind that GPS measures straight line distances - think of knots vs. MPH - If your climbing a 6% grade you can expect roughly a 6% "error" on your GPS (simplified) - HTH (Message edited by niles500 on August 18, 2008) (Message edited by niles500 on August 18, 2008) | ||
| Luvrbus (Luvrbus)
Registered Member Username: Luvrbus Post Number: 504 Registered: 8-2006 Posted From: 74.32.83.140 Rating: N/A |
Nick,you pay for that accuracy in survey the work station I used while I was in business for that type accuracy was about 20 grand the handhelds will get you within 3 to 16 ft now for 200 bucks (Message edited by luvrbus on August 18, 2008) (Message edited by luvrbus on August 18, 2008) (Message edited by luvrbus on August 18, 2008) | ||
| Roderick W. Chandler (Rod)
Registered Member Username: Rod Post Number: 37 Registered: 11-2003 Posted From: 69.30.188.128 Rating: N/A |
I have done several trips from Texas to North Carolina and the speed on the GPS when compared to distance traveled and time elapsed was right on. Speedometer usually indicated 3-5 miles faster than GPS. | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 720 Registered: 11-2005 Posted From: 208.54.200.113 Rating: N/A |
My Garmin Nuvi 200 is right on, I depend on it much more than the bus speedo. | ||
| Luvrbus (Luvrbus)
Registered Member Username: Luvrbus Post Number: 505 Registered: 8-2006 Posted From: 74.32.83.140 Rating: N/A |
Zimtok,gps speedometers are very popular in boats I have saw one in a GM bus. | ||
| GlennVin (Glennvin)
Registered Member Username: Glennvin Post Number: 36 Registered: 10-2002 Posted From: 75.117.20.194 Rating: N/A |
My Tom-Tom: * is right on the money in the bus * is 2 MPH faster than my GMC pick-up * is 3 MPH slower than the Jeep Cherokee So, this should tell you...nothin' LOL Glenn Tallulah Falls, GA 84 MCI-9 | ||
| James Robinson (Jjrbus)
Registered Member Username: Jjrbus Post Number: 151 Registered: 12-2000 Posted From: 68.242.55.148 Rating: N/A |
Oh great, you guys are tough. I used my Streets and trips to set the new speedometer in my bus!!!! I wonder how far its off????? | ||
| FAST FRED (Fast_fred)
Registered Member Username: Fast_fred Post Number: 442 Registered: 10-2006 Posted From: 208.100.193.160 Rating: N/A |
Sure your in Statute not nautical mph? Cars are BUILT off by 5 to 10% so the mileage will look good . FF | ||
| Austin Scott Davis (Zimtok)
Registered Member Username: Zimtok Post Number: 278 Registered: 9-2006 Posted From: 216.37.73.226 Rating: N/A |
Well I guess splitting the difference is a good rule of thumb here. The GPS shows the bus doing a top speed of 62-63 mph on a flat run. (66 on a downhill wooo hooo) Cars following me show it to be 65-66 of a flat run. I'm not going to worry about it unless I get into an area where the cops are very stickey obout speed. Then I'll just side on caution and make sure I'm slower then the speed limit. It is very easy to come into my town after a gig at 3am and be running a little fast. The speed limit drops from 65 to 55 to 45 in about 1/2 mile. . | ||
| Jack Conrad (Jackconrad)
Registered Member Username: Jackconrad Post Number: 805 Registered: 12-2000 Posted From: 68.26.110.161  Rating: N/A |
If you want to check it, the next time you are on an intersate, drive so GPS indicates 60 MPH. Time yourself between 2 mile markers. Should be exactly 60 seconds. Jack (Message edited by JackConrad on August 19, 2008) | ||
| Skip N (Skip)
Registered Member Username: Skip Post Number: 28 Registered: 11-2006 Posted From: 161.7.91.35  Rating: N/A |
Jack, Depending where you are mile markers are not a mile apart. If there was a realignment the road crews just put it as a best fit. Very few states remilepost a whole route for an alignment change. Interstates seem to be the most static from changes. Some states remilepost the whole route on a every 2 to 3 year cycle but that can cause addressing problems for some e911 systems. YMMV Skip PS <edit add> as a general rule GPS accuracy is 1.5 greater error rate on the elevation than the horiz. Both DMI and odometers measure in a relative 3d enviornment. Some states design and put there MM on a 2d system from stationing offsets. Some states place there MM using a DMI and just drive down the rode and place the MM. DMI's have their own accuarcy issues  FWIW (Message edited by skip on August 19, 2008) | ||
| Luvrbus (Luvrbus)
Registered Member Username: Luvrbus Post Number: 508 Registered: 8-2006 Posted From: 74.32.83.140 Rating: N/A |
Some states have a special speedometers test areas on the highways | ||
| Jim Wilke (Jim Bob) (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 296 Registered: 2-2001 Posted From: 69.77.156.190 Rating: N/A |
Fred, I understood that the car companies made the speedo's about 2-3 MPH fast so you don't get tickets due to inaccurate speedo. Folks would sue them if that happened. Most people have no clue how to figure mileage anyway. (Before you fight me on that one, ask 50 people at random what their fuel mileage is. 90% will say something like "I get great mileage, I can go 345 miles between fill ups." or something to that effect.) We are not normal, my friend! | ||
| Bruce Henderson (Oonrahnjay)
Registered Member Username: Oonrahnjay Post Number: 264 Registered: 8-2004 Posted From: 72.12.39.78 Rating: N/A |
You're exactly right, Jim Bob. Speedo's are made to be 2-3 mph fast for "variation" - another factor is that car companies want to be flexible on the maker of tires that they buy and put on the car. So, the speedo is designed to leave a little "wiggle room". And there is the issue of tire wear changing the speedo calibration. If you make it 2-3 mph fast on purpose, you're very unlikely to see it read too low, even with all the variations. Typically the *odometer* is more accurate than the speedo, so when you're figuring fuel economy, the reading on the odo is pretty reliable. Oh, and my last tanks' MPG's were: 56.36 60.84 62.64 54.06 55.22 (VW Jetta Wagon - the bus is on jack-stands getting wheels painted, new tires, and a brake rebuild) (Message edited by oonrahnjay on August 19, 2008) | ||
| FAST FRED (Fast_fred)
Registered Member Username: Fast_fred Post Number: 443 Registered: 10-2006 Posted From: 208.100.193.28 Rating: N/A |
Depending on the GPS the "refresh rate" can be changed, go as short as you can. FF | ||
| Jim Wilke (Jim Bob) (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 298 Registered: 2-2001 Posted From: 69.77.156.190 Rating: N/A |
Bruce, my better half is driving a Jetta TDI sedan. Traded the old Navigator for it about 4 months ago. Can't beat the mileage & its a fun zippy car to drive. Our second Jetta Diesel. It can also be towed four down without modification. | ||
| Bruce Henderson (Oonrahnjay)
Registered Member Username: Oonrahnjay Post Number: 265 Registered: 8-2004 Posted From: 72.12.39.78 Rating: N/A |
Jim, you may remember that you sold me a tow-plate for my Jetta. That one was a diesel - totalled by a drunk running from the cops at about 115 MPH. Unfortunately, I was in the car and driving it at the time. The car rolled at least once side to side, landed in the ditch upside (one policeman told me that the mark of the roof sliding was over 60 feet long), then flipped end over end and landed on its wheels. I flipped off the ignition (car was still idling), and crawled out of the passenger's side (car up against the ditch bank on the driver's door) helped by a police sergeant. There were 7 police cars around me. As I was brushing myself off and looking for blood, a deputy sergeant came striding up, stuck a Glock in my face and said "Put your hands where I can see them". I said to myself "I've been through all this and Barney Fife is going to shoot me on the side of the road" but then the sergeant came running back up and said "Put that thing away - he's a *victim*". (They had already handcuffed the drunk but not before he'd added "hit and run, resisting arrest, and assault on a police officer" felonies to the three that he's already wracked up). I had a broken toe (hit the back of the clutch pedal rolling over?), a badly wrenched back (hit the center console?) and some big seatbelt bruises. If you walked up behind that car on the driver's side, first thing that your knee would have touched was the left rear tire -- and it was a Jetta not a Golf. Your towplate was badly bent and damaged from being dug into the ditchbank. After that little diesel (which had averaged nearly 57 MPG over it's 130K life) saved my life, I bought another one - a TDI wagon this time -- and added a new towplate! (Message edited by oonrahnjay on August 20, 2008) | ||
| RJ Long (Rjlong)
Registered Member Username: Rjlong Post Number: 1409 Registered: 12-2000 Posted From: 67.181.166.160  Rating: N/A |
Jim - Is the wife's TDI a five-speed, or an automatic? | ||
| Mel La Plante (Mel_4104)
Registered Member Username: Mel_4104 Post Number: 58 Registered: 7-2006 Posted From: 154.5.117.64 Rating: N/A |
the way that i checked mine was by going to the police station and asking the fellow in charge of highway patrol if they would mind checking it using their radar and my microsoft street and trips,he said sure and told me where they would be working,so when i blinked my lights we recordrd our readings--radar-gps-speedo. the radar and gps were .5 miles off and the speedo was 4.8and 4.6 off of the return run. the officer said thanks as he wondered how close radar is as to the gps, and we talked about the the chance of error between different makes of gps and he said yes that you get what you pay for when buying one but he said the micro soft street and trips seemed very accurate. | ||
| Jim Wilke (Jim Bob) (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 300 Registered: 2-2001 Posted From: 69.77.156.190 Rating: N/A |
Bruce, I had forgotten just who I sold that baseplate to. I can't believe that little car did so well protecting you but I know that VW builds great cars & is very focused on safety. Glad to hear you came out of it O.K. RJ, both of ours have been stick shift. The automatic gets lower mileage and can't be towed without modification. | ||
| Jack Conrad (Jackconrad)
Registered Member Username: Jackconrad Post Number: 806 Registered: 12-2000 Posted From: 76.3.173.51 Rating: N/A |
We run both Streets & Trips and Delorme at the same time on our laptop (split screen left half-Streets & Trips, right half-Delorme) I have found speeds are always the same, but elevations are not even close. Delorme (WAAS enabled, whatever a WAAS is) is much closer. Streets & Trips shows our house as being 30-40' below sea level. Our place is about 14-17' above sea level, which is usually close to what Delorme shows. Here is a question: Not that it makes any difference, but where is my GPS measuring the elevation? The antenna is under the fiberglass cap (12' above the ground) while the laptop is on a dash bracket about 6' above the ground. Jack | ||
| Luvrbus (Luvrbus)
Registered Member Username: Luvrbus Post Number: 515 Registered: 8-2006 Posted From: 74.32.83.140 Rating: N/A |
Jack, a gps takes a reading of the ambient air pressure for elevations how it works on a lap top is a mystery to me.WAAS is a system of satellites and ground stations to correct the signals I don't know about today but at one time WAAS was available only in North America this is the reason why the gps equipped with WAAS is more accurate than one without (Message edited by luvrbus on August 21, 2008) (Message edited by luvrbus on August 21, 2008) | ||
| Bruce Henderson (Oonrahnjay)
Registered Member Username: Oonrahnjay Post Number: 267 Registered: 8-2004 Posted From: 72.12.39.78 Rating: N/A |
Jack Conrad wrote "elevations are not even close. Delorme (WAAS enabled, whatever a WAAS is) is much closer." That makes me feel safer! WAAS is a special altitude algorithm for GPS that is meant to greatly increase altitude accuracy. It was designed to replace the ground-based radio transmission of altitude for aircraft making an instrument approach (for pilots, it replaces the glideslope function of an ILS -- combined with GPS for the localizer function, it's possible to fly an "ILS" into an airport with no ground-based ILS system or even into a point that's not an airport). The GPS figures altitude by triangulation of the satellites (OK, multiple triangulation of many satellites for nit-pickers). Early GPS that was designed mainly for two-dimensional location didn't have a very good altitude function. For both military and civilian purposes, those days are waay gone. BH NC USA | ||
| Skip N (Skip)
Registered Member Username: Skip Post Number: 29 Registered: 11-2006 Posted From: 161.7.93.2 Rating: N/A |
Jack, Measurement is a time based function. Part of what the satellite sends in its packet is the transmission time then your computer (GPS) compares that to the time it has. From that it starts the triangulation code. That is what happens when the GPS syncs up. Basically sets the clocks to run at the same time as the satellites. So in a way your position is maeasured at the computer because that is where the time is proccessed. FWIW YlocMV Skip | ||
| Jack Campbell (Blue_goose)
Registered Member Username: Blue_goose Post Number: 102 Registered: 5-2007 Posted From: 71.100.201.35  Rating: N/A |
My garman is measured at the antenna. You can check to find out how close it is in the menu. It depends on how meny sat. it has and which ones they are. Jack | ||
| FAST FRED (Fast_fred)
Registered Member Username: Fast_fred Post Number: 446 Registered: 10-2006 Posted From: 208.100.193.111 Rating: N/A |
After that little diesel (which had averaged nearly 57 MPG over it's 130K life) Guess its time to tune the Diesel Rabbit , we only get 46 or so. FF | ||
| Jack Conrad (Jackconrad)
Registered Member Username: Jackconrad Post Number: 809 Registered: 12-2000 Posted From: 76.3.173.51 Rating: N/A |
Thanks guy for the education. It is not a good day unless you learn something new. Jack | ||
| RJ Long (Rjlong)
Registered Member Username: Rjlong Post Number: 1413 Registered: 12-2000 Posted From: 67.181.166.160 Rating: N/A |
Fred - "Guess its time to tune the Diesel Rabbit , we only get 46 or so." Not sure if you were being tongue-in-cheek here, but the TDI engines use a different injection system that's electronically controlled, not mechanically like your Rabbit Diesel. They're also turbocharged, putting out more than double the hp and torque, not to mention a whole lot less smoke. And to bring this back on topic, my first Garmin GPS (a C330) seems to be extremely accurate. Just got a new Garmin Street Pilot 7200 via FedEx today, will see how accurate it is during my upcoming road trip this weekend. . . FWIW & HTH. . . |