At highway speeds my Nuvi 760 conistently reads 5 MPH slower than my speedometer...which is more accurate? But, really officer, my GPS had me going just 65
Tires also may be underinflated. Actually changes in tire size should produce proportional ratio GPSspeed/CarSpeedometer. If the readings are identical at lower speed, then it's probably wrong car speedometer.
Last edited by vleek on Tue Feb 19, 2008 10:44 pm; edited 2 times in total
Joined: 15 Dec 2007 Posts: 939 Location: Central PA (FN10) - USA
Wed Feb 20, 2008 3:53 am
To check your speedo why not just time yourself while on an interstate against the Mile Markers? At 60 mph it should take 60 seconds. You do the rest of the math. You just need to find a stretch where you can actually go just 60 for a short time without getting rear-ended or a lot of single digit salutes.
I'd take Spyder63's suggestion and I wouldn't go by the GPS. I've taken four GPS devices, put them on the dash, driven a consistent 60 mph on the Interstate and they all showed a different (yet steady) speed.
With 24 orbiting satellites using triangulation, and GPS speed being calculated using an atomic clock, not some old worn out treads on those tires … Guys, forget about speedos on your car … Only laser guns can claim to compete with GPS accuracy and even them have lost in court.
I totally disagree with the results of that article, unfortunately. GPS has a huge margin of error that can't be ignored in cases like that. If they are so accurate then why do multiple GPS devices in my car each show a different speed?
I totally disagree with the results of that article, unfortunately. GPS has a huge margin of error that can't be ignored in cases like that. If they are so accurate then why do multiple GPS devices in my car each show a different speed?
You're probably right - I just came across it by accident, thought I would pass it on. One reason why multiple GPS devices in your car each show a different speed may tie in with differences in technology, like the number of correlators for example and I'm sure you can think of other features that one cause one to perform differently than the other.
I can understand how a speedometer could vary but I don't understand how a GPS could unless they calculate speed using slightly different formulas. (Which would be odd.)
I've clocked my speed using:
My speedometer
Previous Lowrance iWay 350c
eTrex Camo
gps12
Previous nuvi 350
Previous nuvi 360
Current nuvi 660
Current Freedom Keychain GPS through my Motorola Q
The only one that gave a different reading was my speedometer.
Not saying your findings are inaccurate, just posting a different result.
I think some of them are perhaps better of taking into consideration their own processing/calculation times and how that might impact the calculated speed. Some GPS devices are also better than others at throwing out junk data than others. But yes, it is weird to be driving down the road and have my speedometer say 60, one GPS saying 61, and another GPS saying 62, and another GPS saying 64.
I had the same thought when I looked at your list and indeed I would expect a similar results from the same brand. I've been comparing my car (Outback Limited sedan) with most of the different brands on the market which is likely why I saw such a discrepancy.
Something else probably related is to look at the speed your GPS shows while it is "at rest". Now many of the auto devices are likely programmed to show 0 when it is close to 0. But for example I just turned on my 760 and have left it in place for the last five minutes and it shows an average speed of 1.7 mph with a max speed of 3.1 mph. That is likely a good indication of how the margin of error of GPS impacts the speed readout.
Yeah, good point there... I know the Freedom can read .1 - .7 mph when I stop. I thought the reason was due to the fact that it was still swinging back and forth on my keychain lol.
@Tim: "But yes, it is weird to be driving down the road and have my speedometer say 60, one GPS saying 61, and another GPS saying 62, and another GPS saying 64."
I'm surprised that your speedmeter showed a lesser number, I would have expected the opposite. It's a known fact that most car manufacturers have cranked up their speedos for a number of reasons, lawsuits for one. I think it's fair to say that that dial is an indicator, NOT a precise measurement; such that if you're showing 110 mph, odds are that you're probably doing only 109 or 108 or 107 - the variance can be as much as 5 mph depending on the manufacturers. There are some exceptions, the 2007 Toyota Camry for example is dead on - why this one and not others! It beats me!
Yea, sorry I had it backwards in that example, my car was reading the highest speed. I'll see if I can go do another test today with a few different devices.
Joined: 15 Dec 2007 Posts: 939 Location: Central PA (FN10) - USA
Wed Feb 20, 2008 11:10 pm
In the state of Pennsylvania only state troopers can use radar and by law they have to give a 6 mph leeway. If you are doing 71 in a 65 zone you are "safe". Seems the legislators are convinced vehicle speedometer are not accurate.
Apparently, the margin of error also varies between units within the same model. I turned my 760 on after reading Tim's post of his experience earlier. Showed no movement in over 3 hours of sitting still.
0.00 mph max speed
Yea, sorry I had it backwards in that example, my car was reading the highest speed. I'll see if I can go do another test today with a few different devices.
Have you had a chance to run those other tests TIM. This is an interesting issue, so was just wondering what you came up with.
Joined: 15 Dec 2007 Posts: 939 Location: Central PA (FN10) - USA
Fri Feb 22, 2008 3:04 pm
Of all of my vehicles I have found that only my Cadillacs always agree with my GPS. The rest vary by 2-4 mph, pretty much on the low side. My '63 Corvair speedo reads WAY high at speeds above 35mph so I always carry a GPS and so far have not been pulled over. For awhile I was impressed with how well it handled at 90-100 mph. Then my bubble burst when I discovered I was only going 65-70!
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