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How accurate is the speed?

Eric K
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My Garmin V is always 3 mph SLOWER than what is shown on the car speedometer. I notice this when I'm on a flat, straight stretch of interstate and have the cruise control on.


I guessed that either the calibration on the car speedometer is a bit off, or that the manufacturers build in a little "cushion" just like the police give us a 5 mph margin of error before writing a ticket.


After all, it's impossible for the satellites to be wrong, isn't it?



What if the Hokey Pokey IS what it's all about?

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My GPSr often reports a speed of as much as 10mph slower than my speedometer. I remember reading somewhere that speedos are legally allowed to report too fast a speed, but not too slow a speed, so when they make them they err on the side of too fast. I think that was referring to British law, but I would imagine it's the same over there.


I suspect the speed your GPSr reports is bang on.

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tire size will affect the accuracy of your cars speedo. manufactures are required to calibrate by the stock size, if you change the size it will affect the accuracy of your speedo. the speedo gear setup is generally a set of plastic cogs which will wear over time and generally dont give good accuracy. here in nl i understand that tickets given for <15 km/hr over the limit have a 80% chance of being overturned for these reasons.


also my mazda truck reads 100 km/hr when my gps say 95 km/hr. i trust the gps for accuracy.

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Our town has one of those trailers that the police department puts out to clock how fast you're going. Our geovan says it's going 3mph faster than what they clocked us at. We also used hand held radios and compared it with a friends van....also 3mph off. In playing with the GPS speed, the GPS also says we're going 3mph slower than what the speedometer says........And since the police are always right icon_wink.gifwe're going to believe the GPS is right on..........

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In the cross country driving trip we did in September I was courious to see how the speed on my GPSV woudlrelate to the speedo in my '02 Envoy. I was surprized to find they matched exactly. I was expecting one or the other to be off. They also have matched the radar speed displays we have passed.



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It's like saying I find my GPS 3 mph faster than my Flinstonemobile and 2 mph faster than my Conestoga wagon. GPSr's are way more accurate than automobile speedos. Auto speedos not only are usually off a mph or two, but also off differently over their range. I have a Sonoma that's about 1 mph under at 30 mph, over 2 at 60 mph, and over 4 at 100mph. icon_biggrin.gif I confirmed this with airplane markers, a stopwatch, and some dusted off algrebra.


Garmin specs say 0.1 mph presumably with full signal. I would guess that it's not more than a factor or two off for lesser signal strength and personally figure it's easily within 1/2 a mile an hour.

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To really test GPS against car speedo...


Find a set of mile posts (most states have them exactly 1 mile apart) get car speedo on 60 and watch GPS..... IF you are going exactly 60 mph you will do 1 mile in 60 seconds...


Compare results...





I'm Diagonally Parked, In A Parallel Universe.


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I have a competition tow boat (for wakeboarding) with dual speedometers which allows me to be highly accurate and precise in my MPH reading. When I compared the boat's MPH reading with my GPS, both were nearly identical. A difference of .3 MPH. I'm convinced that the GPS speed is quite accurate.

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I've compared my 12MAP against my radar unit, which is routinely calibrated and certified to be off no more than +/- 1 MPH. My 12MAP is always right on. Speedos in vehicles can be off a great deal depending if size of tires have changed or gear ratios. Usually a change in 1 tire size will change the speed by 3-5 MPH.

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I've also found that not only is the GPSr more accurate than the speedo (verified by the officer who wrote the ticket), but the speed will vary quite a bit (3-4 mph) while speedo reading is the same speed. I would guess that this results from a fat needle and skinny marker lines...


Children today are tyrants. They contradict their parents, gobble their food, and tyrannize their teachers. - Socrates

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