And GPS speed only works when it can “see” multiple satellites. That fails in cities with high buildings and cloudy days.
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Yes. When travelling from Houston to Biloxi one year while a hurrican was in the Gulf of Mexico my GPS was totally out of commission.
Also fails on steep hills
Interesting. I have not experienced that. Why do you think that is? Blocked radio transmission path or something else?
GPS determines speed on vertical movement. It has no way of knowing if you’re going up hill.
My Garmin Nuvi 2595 has elevation. I don’t think it is terribly accurate but it does know changes in elevation.
My garmin 2595 has never failed to show speed. Not sure how accurate elevation is.
So that means that I was actually going faster than the 40 mph my handheld GPS recorded while skiing downhill?
That’s funny.
I meant GPS doesn’t take in account you vertical speed. Only the horizontal speed.
I had a hand held GPS record a peak speed once that I could not possibly have attained while skiing downhill. I downloaded my GPS tracks into the computer and it showed me making a huge zig zag to a point off the trail and then on again that I never made. It was an area that was impossible to go to on skis.
I guess it got a bad signal or something.
GPS calculates your position in a 3D space centered on the center of the earth, based on signals from four or more satellites. The receiver converts this to latitude, longitude and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to mean sea level.
One way it can read speed is to take two successive position readings and calculate velocity from that. This would be an absolute velocity vector. How the receiver converts that to speed along the ground is up to the receiver.
An alternate is to use doppler readings on the signals to calculate speed more directly and more accurately.
Which is used depends on the design of the receiver.
@Mustangman Math was never one my strong points but I see what you mean by the difference between larger & smaller tire’s.How would you explain the dirrerence I was seeing between the speedometer & gps. I do not know what tire size was oringal for truck but it now has p23575r15 goodyear wranglers on. The tires were on truck when I got it still have 80% of tread left. As far as gps single goes I live in a very rural areia with no tall buildings & I have only a very few times have I lost signal due to clouds or trees on a few back roads. I got to where I can judge within 5 to 10 miles of speed by the tach.
The last tires I bought for my Yaris were 195/60 instead of the original 185/60 and the first thing I noticed was that my GPS and speedometer no longer agreed. It’s not a huge error, just a couple of mph at freeway speeds. I now use the GPS odometer to figure my gas mileage instead of the car’s trip odometer.
Typically, the GPS will indicate around 425 miles when the car’s trip odometer indicates 400.
If I’m not mistaken, an absolute velocity vector will always be equal or less than the speed, depending on the deviation from a straight line from point A to point B.
Speedometers have quite a bit of error built in. Usually to the conservative side by design. Read 73mph while actually going 70. That’s slightly less than 5% error but to the good side as far as police are concerned. The speedo only reads the rotational speed of the transmission output and converts it to MPH based on the final drive’s gear ratio and tire diameter. Increase that diameter and the truck goes faster but the speedo reads the same. Smaller tires would read slower. Not sure why your truck reads high with bigger tires. Lots of places for error to creep in depending on your speedo’s design.
Your GPS speed should be pretty accurate with flat open terrain. I’d use that value to “calibrate” how fast you drive. I’ve double checked most every car I’ve owned by milepost markers and a stopwatch. I.e. 60 mph means a mile marker every 60 seconds - makes the math easier.
The magnitude of the velocity vector equals the speed, by definition.