GPS accuracy...

[Mark_88]

So I got lost the other day...as per usual...and by some freak of nature I did a look up on my phone for the location and accidentally turned on my GPS (that I didn't realize I had enabled in the first place)...

And I'm doing this while en route and look at my phone to see that it is actually indicating my speed...in Mph...so I decided to take it up a notch and I was doing 100 Kmph on a highway as indicated by my speedometer...but the GPS is reading 71 Mph...wtf?

I tried varying the speed to see how closely it was following me and it would drop speed as I dropped speed...it would increase as my speedo increased...so it was pretty much up to the second...

The problem is that it was approximately 10 Kph above what my speedo was reading. No big deal, except I've used those roadside speed indicators that the police put out to tell motorists that they are about to set up a costly speed trap if we keep going over the limit (I'm sure those things report back to them to let them know where the money is...lol)...and my speedo has been pretty much right on with those machines...

My question would be..."Does the GPS over indicate ground speed"? I would think it might be out a bit...but 10 Kph is higher than I'd expect...

 

[88_Eddie]

i've noticed that our GPS reads faster than my speedometer, not by much though. if the GPS says i'm doing 80, the speedo will say 78 or so. not sure which one to trust.

 

[Mark_88]

Ah, thanks...when I went up a tire size I figured my speedo was out but never bothered to get it checked...been using the roadside indicators whenever possible just to make sure it's not too far off...

 

[Explorin94]

Mine reads 4mph faster what my speedo says in my 150. I actually follow the GPS reading over the trucks speedo, I don't know why but I do.

On another note....if I follow my GPS to what it indicates where my house is it takes me a 1/4 mile south from where I actually live.

 

[alwaysFlOoReD]

My Garmin gps reads 94 when I'm doing 100 and corresponds to the radar signs.

Richard

 

[6.2]

Any ford I've been in with my gps, the gps shows 4-5 mph slower than speedo. Ford does this on purpose from factory. No idea why. If it shows 60 mph on the speedo I'm doing 55-56.

 

[Ozwynn]

federal law allows a 4% error in the odometer in a passenger vehicle. In a big rig this is not the case and my GPS is only 1 mph different than my speedometer and 75 mpg (as read on the GPS) corresponds with my governed speed....... when I am bumping the governor I am showing 74 on the speedo but the odometer reads the same as the mileage traveled feature on my GPS so I am sure its a mechanical error and the GPS is accurate.

 

[upnsmoke]

federal law allows a 4% error in the odometer in a passenger vehicle.

Would this be applicable to speedometers as well? Do you know when this law came into effect? I only ask because I have driven a number of vehicles in past years whose speedometers I am fairly certain were fast by over 5 percent. The most accessible and accurate way that I have found to check a speedometer is a trip to the drag strip. The only vehicle that I have ever owned that came with an accurate speedometer was a Yamaha motorcycle, which was within one percent of actual speed. I have checked the speed measurements of several gps units as accurately as I am able, and found them to be within 5 percent of actual speed, all reading fast.

 

[doorgunner]

did you know if you drive around ina a tight circle at 30mph....the gps shows you're only doing 5mph.........same thing with going up a steep hill...


seems like gps' are set for straight/level driving only.....


but since I have an aftermarket electronic speedometer....it reads within 1/2 mph of the gps on strate/level roads

 

[upnsmoke]

did you know if you drive around ina a tight circle at 30mph....the gps shows you're only doing 5mph.........same thing with going up a steep hill...

Now this I will have to try.

 

[doorgunner]

did you know if you drive around ina a tight circle at 30mph....the gps shows you're only doing 5mph.........same thing with going up a steep hill...


seems like gps' are set for straight/level driving only.....


but since I have an aftermarket electronic speedometer....it reads within 1/2 mph of the gps on strate/level roads

For entertainment purposes only...... UPINSMOKE!

 

[upnsmoke]

For entertainment purposes only...... UPINSMOKE!

Understood, but I'm still trying it.

 

[straycat]

I don't have a GPS. But this morning I took a buddy to pick up his new Jeep Wrangler and he has a GPS on his phone. I told him about this post and he wanted to try it out. He turned it on so I could see the speed I was doing to compare it as you guys did. I hit 60 in my Ranger (the speed limit) and the GPS said I was doing 59. I went to 65 and it said I was doing 64. I slowed down to 50 and it said I was doing 47.

 

[Ozwynn]

Would this be applicable to speedometers as well? Do you know when this law came into effect? I only ask because I have driven a number of vehicles in past years whose speedometers I am fairly certain were fast by over 5 percent. The most accessible and accurate way that I have found to check a speedometer is a trip to the drag strip. The only vehicle that I have ever owned that came with an accurate speedometer was a Yamaha motorcycle, which was within one percent of actual speed. I have checked the speed measurements of several gps units as accurately as I am able, and found them to be within 5 percent of actual speed, all reading fast.

I don't know when the law came about.... just remember reading a law suit against Honda because their odometers exceeded the 4% error. I know because of road tax big rig odometers need to be accurate within 1%. I believe the error also applies to speedometers..... in both cases.....

It came about because of changes in tires size as the tire wears...... tire manufacturers claim it to be a total of 8% so you get 4% either side.

 

[CHKNFKR]

Depends on the gps. I've used some that couldn't even find the county i was in. Most cell based ones have a tendency to to get confused when you stop moving or show you being a block over from where you actually are. the one i use now reports back how many satellites it's communicating with and estimates its own accuracy. typically it reports something in the 4'-8' range. it also loads topographical data and factors in changes in altitude to report back accurate speed and location data while climbing and descending hills. It's within 1% of the roadside speed indicators and johnny laws radar gun.