has anyone tested our speedo accuracy with gps? i am curious to know if i was really going 129 mph about 10 minutes ago. the speedo in my last car was off by at least 5 mph at speeds over 110, and became less accurate as the speed increased.

 

Kinda curious as to how GPS can prove how fast you are going?

 

Well if the wheels and tires are just an inch in circum. off that will give a 2mph difference at the speeds you are going. Then of course as you get faster that difference will multiply.
I wonder if the gps reading is delayed at all because of the distance in travel of the signals.

 

a GPS can calculate your speed by taking 2 readings from the satellite. It uses the distance you traveled between the 2 points, and divides that by the time between the readings. do some conversions, and voila, mph

 

My girlfriend has this garmin PDA/GPS and it shows your speed and its fairly accurate... We have had it in 3 different cars and it is right within 1-2mph.

 

GPS is much more accurate than a car's speedo for many reasons but that being said, the TSX's speedo is pretty good. The reason why your speedo is less accurate at higher speeds is that your wheels tend to balloon out and will give you a slightly higher effective ratio.

 

My GPS claims my TSX's speedo is off by 4mph. The speedo would say 71mph, but in reality it would be going 67mph according to my GPS.

 

I see what you're sayin...but the accuracy of GPS to pinpoint your location could be off by as much as 1/2 mile...So if you were off by.5 on both sides...that's one mile, could not be all that accurate.

FYI.....I use 1/2 mile because I would think GPS is GPS, and when designing approaches for aircraft, we calculate an error of .5 miles for actual aircraft placement.....if that makes sense.

 

Garmin quotes a 0.1kmh (0.06mph) accuracy from 0-1874kmh in the literature I have.

 

Hey Dan, not to hijacl this but what Garmin product do you own and what software are you using. I've heard they have a new canadian map which is vastly better than the last.

 

1874kmh is pretty damn fast......Maybe depends on type of GPS interrogator on the ground/ car/ whatever? Some must be more accurate than others I guess... Makes you feel safe when flying though, huh, being off by a half mile....

 

I've got a bunch of Garmin stuff but I use the iQue in my TSX. AFAIK I have the latest maps in there but you can always get new ones. This mapset even shows the little dirt road that goes around my lake at the cottage so I don't know how much more detailed it can get.

A lot of the new burbs are in there too even though they don't exist yet or are in the process of being built. They must have to file the street plans long in advance of the actuall construction or something.

 

I'll borrow my girlfriends and crank it up to 100 and let you guys know

 

GPS 101

Location wise, GPSRs (GPS receivers) can be accurate to within 10 ft in all directions (including altitude) - with some trickery it can be even better than that - that's how one can put a bomb dropped from high altitude (or in the case of a Tomahawk missle, several hundred miles away) through a certain window of a building (getting the building right is another matter).

Speed is another matter - unlike a location which is a direct measurement, speed is a differential measurement and this is why Garmin and many other GPSR manufacturers can claim speed accuracies much better than 1mph. (the opposite is true in a typical car - speedos are almost always high by a few % while the odometer has to be accurate to within less than 1% - gov't mandate)

Other than a very accurate timer setup between two fixed points, the speed displayed by a GPSR is the most accurate speed indication you can get (better than the radar gun that LEOs have) .

Virtually everyone gets their detailed map data from NavTech. The general map data comes from different sources. In the case of Garmin, rural Canada is sometimes better served by the general underlying base map built into the GPRS rather than the detailed map given by their MapSource cartography.

If you have a Garmin GPRS and you are in the boonies of Canada (which means as soon as you drive out of Toronto), turn off the detailed map - you'll be pleasantly surprised by what you find in the base map.

 

mm, guess i'll just have to make another run and test it myself someday. great info all around, guys!

 

The other day I was driving through one of those "here's how fast you're going" setups with the mobile radar cart. I was dead on 70 (intentionally because I knew it was there) and the cart registered me as going 70. There was nobody else around so I'm sure it was me. Interesting to know it was dead on at 70.

 

I drove by a speed limit sign that had a speed gun and display. It said I was doing 81mph in a 55 zone. I looked down and, indeed, the pointer was just a tad over 80. Hence, I think it's pretty accurate.

 

I wish they had those signs here. They sound like a challenge more than anything.

 

Bingo. As long as those things don't have a camera....it's full throttle until I pass.

 

Speedometers are generally less accurate up in the higher ranges because they all have a certain level of error and the larger the number the larger the error.
Also, you're running a 19" wheel, and 35 series tires. They're according to my calculations about a quarter of an inch (6mm)smaller in diameter which would mean your speedo will read slightly faster than you're actually moving.. which will also contribute to your seeing a speedometer error as well.

(.35x225) x 2 + (19 x 25.4) = 640 mm
(.50x215) x 2 + (17 x 25.4) = 646 mm Stock

 

OT:

Those signs work well if you have stopped accelerating, acceleration is near zero. But if you accelerate towards one of these signs, you will get them to give readings that are way above your speedometer's reading. I did a hard acceleration to about 20mph from zero and the sign had me going 55mph.

 

It's way more accurate than 1/2 mile, especially after the US govt disabled selective availability. On a bad day, 4-5 satellite lock, obstructed sky, the unit indicates about 30m accuracy. On a good day, 10-12 satellites, clear skies, 2-3m accuracy. Somtimes, I've literally turned into the driveway of my target address as the distance hit zero.

My only concern is that during accel/deceleration, the update speed of the GPS may not keep up. On my unit (eTrex) it's about 1 update/sec. For steady-state conditions though, it's impressively accurate.