GPS accuracy...or randomness

Old 03-29-2017, 08:07 PM
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Default GPS accuracy...or randomness

Been thinking for a while if my speedo is accurate, since I have bigger than stock tires with the stock speedo gear. I do not have a gps, but I do have a smartphone with google maps. Just curious how accurate y'all think google maps, or any other free gps service is? Tonight I drove 28.8 miles according to my phone, and only 26.8 on my jeep. If this is accurate, I can get around 240ish per tank according to the jeep. Which would be around 257ish actual miles. Obviously, I know my speedo is wrong. Anyway I'm going to track my miles via google maps just to see how many miles it's off. Do y'all think google maps is an accurate way to track miles? I know it's an XJ and mpg isn't on the top of the list haha, just bored and curious.
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Old 03-30-2017, 01:47 AM
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It is pretty accurate. Even if its off a mile or two over the course of a full tank of gas, you will be in the ballpark. Why not just get the correct speedo gear?
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Old 03-30-2017, 05:59 AM
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Speedometer app with a trip odometer will take care of that for you. Then you can calculate the correction factor, or you know, buy a new speedo gear.

Unless you put 235/75R15 where 225s were, your speedometer is off.
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Old 03-30-2017, 06:35 AM
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I checked my speedo with the local police stationary radar and it reads 2 mph lower than my speedo does. I haven't checked to with a stopwatch over a measured mile yet, but I will.

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Old 03-30-2017, 06:52 AM
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Speedometer can legally be off 20% and bigger tires with out the right speedo gear can throw it off also.
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Old 03-30-2017, 06:55 AM
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Originally Posted by andrewmp6 View Post
Speedometer can legally be off 20% and bigger tires with out the right speedo gear can throw it off also.
I think you added a 0 to that dude...
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Old 03-30-2017, 07:02 AM
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How are we going to change that when there is no speedo gear, like in my WJ? The PCM reads wheel speed and output shaft rpm.
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Old 03-30-2017, 07:11 AM
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Originally Posted by dave1123 View Post
How are we going to change that when there is no speedo gear, like in my WJ? The PCM reads wheel speed and output shaft rpm.
On those vehicles the PCM can be reprogrammed with a correction factor. I think the OBD-II XJs can, too, but the gear is cheaper (although less accurate)

So your WJ uses the ABS sensors?
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Old 03-30-2017, 07:24 AM
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Originally Posted by salad View Post
Speedometer app with a trip odometer will take care of that for you. Then you can calculate the correction factor, or you know, buy a new speedo gear.

Unless you put 235/75R15 where 225s were, your speedometer is off.
Speedometer app..good idea. Do you have one you recommend?
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Old 03-30-2017, 09:48 AM
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Originally Posted by Danger12 View Post
Speedometer app..good idea. Do you have one you recommend?
On my old iPhone I had one named "Speedometer" lol
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Old 03-30-2017, 12:27 PM
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Originally Posted by salad View Post
So your WJ uses the ABS sensors?
Yup! The rear ones at least. That's from the FSM, and there's a hall-effect switch the reads the notches in the park lock ring. I wonder what it does in low range?

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Old 03-30-2017, 12:32 PM
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Originally Posted by Danger12 View Post
Speedometer app..good idea. Do you have one you recommend?
i have one called Ulysse Speedometer. i have verified it against an actual GPS. pretty accurate
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Old 03-30-2017, 12:36 PM
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And everybody thought putting those satellites up there was stupid! Actually it was for the military!
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Old 03-30-2017, 12:54 PM
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Originally Posted by dave1123 View Post
Yup! The rear ones at least. That's from the FSM, and there's a hall-effect switch the reads the notches in the park lock ring. I wonder what it does in low range?
Interesting. Does the PCM know it's in low range? Or maybe it just ignores the park lock ring and uses the wheel sensors.

Originally Posted by dave1123 View Post
And everybody thought putting those satellites up there was stupid! Actually it was for the military!
Yup - I remember when they finally let the public market at the algorithms to give precise calculation. Previous to that I think the locations were approximate to 2 meters
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Old 03-30-2017, 08:31 PM
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Yeah, it got better after they got the whole satellite net up.
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