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Discussion Starter #1
In case anyone is wondering what the speedometer percentage error is on the IS I did a little test to find out:

Equipment:
Garmin GPS III+
'02 5_Spd (20k on factory tires)

Parameters:
Cruise control set at 100MPH
Gave GPS 6-8 seconds to catch up before recording speed

At exactly 100MPH, the GPS read 97.5 Mph. So there is approximately a 2.5% built in error on the speedometer. Not too bad.
 

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IS_5SPD said:
In case anyone is wondering what the speedometer percentage error is on the IS I did a little test to find out:

Equipment:
Garmin GPS III+
'02 5_Spd (20k on factory tires)

Parameters:
Cruise control set at 100MPH
Gave GPS 6-8 seconds to catch up before recording speed

At exactly 100MPH, the GPS read 97.5 Mph. So there is approximately a 2.5% built in error on the speedometer. Not too bad.
what's the accuracy of your GPS system?
 

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Discussion Starter #5
The GPS is programed to average out it's error....There shouldn't be more than .1 +/- MPH error with it. GPS's in general are very accurate. During the time of the test, the EPE (Estimated Position Error) as reported from the GPS was only 13 ft. Additionally, I had 7 satelites locked in, which is more than enough to get an accurate reading.
 

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I would think three satellites is best...? Like the old proverb about the man with two watches never knowing what time it is. :)
 

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what type of GPS system, i'll need to use one of these pretty soon in a test

-gte






IS_5SPD said:
In case anyone is wondering what the speedometer percentage error is on the IS I did a little test to find out:

Equipment:
Garmin GPS III+
'02 5_Spd (20k on factory tires)

Parameters:
Cruise control set at 100MPH
Gave GPS 6-8 seconds to catch up before recording speed

At exactly 100MPH, the GPS read 97.5 Mph. So there is approximately a 2.5% built in error on the speedometer. Not too bad.
 

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Speedometers are designed to be most accurate at (legal) freeway speeds. If the speedo said 100 and the GPS said 97.5, that would be very, very good. Why don't you try running past a state trooper at an indicated 100mph and see what he tells you :)

-Matt
 

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Discussion Starter #9
Matt.....actually it's the other way around.....The faster you are going, the further off the speedometer gets. For example, at 40 mph it would only be 1 mph off; at 120 3mph off. All in all, thats not too significant. My Honda CBR bike is calibrated at approx 8% faster than actual speed.
 

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It's not the other way around, and here's why. When the DOT approves vehicles to be sold in the United States, there are a myriad of things that the vehicle has to pass as far as testing. One of them is speedometer accuracy.

Since it's important to have a speedometer measure accurate speed, the government (and therefore the manufacturer to its supplier) set what are called "allowable limits" for various speed increments. The strictest increment is 50-60mph. So when speedometers are calibrated, they're done so that they're most accurate at highway speeds. Call it an inverse bell curve.

Typically, speedometer margin of error is +/- 4 to 5mph at 20 and 100mph, +/-3mph at 40 and 80mph and +/-2mph at 60mph.

This margin of error is set for air-core speedometers, and not the old cable-operated ones. (Remember the old 80s GM sedans with the huge 0-85mph speedometer? The needle would shake from 0-40mph, but be relatively still at 55mph.)

-Matt
 

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All new 12 Channel GPSs are accurate with the speed. I have the cheap yellow Garmin eTrex and have done similar tests. At steady state, it is accurate to +/- 0.1 knots, or about +/- 0.12mph. Update rate is once per second so after 2-3 seconds at steady state I notice it has stabilized. Here were my results. Anyspeed under 60mph was exactly correct. I only checked 70 and 80MPH which the speedo was under by 1 and 1.5mpg respectively.

If I were to buy another one, I'd get the eTrex Venture for about $140-150 from Amazon. It has WAAS for better accuracy (in location not speed) and the LCD has better resolution.
 

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That is true, what the article says, my old company car was a chevy lumina. The speed was always dead on with my GPS. The ford I drive now, is on average about 5 miles off.
 

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Discussion Starter #14
Good articles......Considering BMW's average error is 4.54% as reported by Car&Driver, the IS's 2.5% error doesn't seem so bad. I agree with their assessment that luxury vehicles are calibrated further off than your typically family sedan....It's good to know that we have an additional margin of error....Just think, when you look down and think you're doing 80 when that cop pulls you over, you'll understand why he say's "I got you going 77 back there".....Hey, that just might save you $100 in additional fines... :) :) :)
 
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