GPS readout vs speedometer

[glider]

should i trust the speed my GPS is telling me or my speedometer?

GPS is telling me my car's speedo is out by ~4km/hr (when speedo says 100, gps says ~96km/hr, speedo at 60 also says 56 on gps etc)

there seems to be no definitive answer on the net

what do you folks think?

[sooty]

I thought our cars were out by about 4km/h at 100km/h as a safety thing...so you're not actually speeding if you sit a bit over etc...so i'd assume your speedo is doing as it's meant to...and your GPS is pretty accurate in your actual speed :)

[Tigra]

I have seen a TomTom GO720 GPS in a certified Highway Patrol vehicle. The speed shown on the GPS was 1k out from the speed that was shown on the radar device.

Having said this, I would still go by the speed on the speedo. Reason is, if you get a ticket, a GPS device will not cover you in court if your going to dispute it. At least you can have your speedo tested by a mechanic to prove your defence.

As far as the 4k's out - I get the same with my device in every vehicle I put it in.

[Huhness]

i get 4Kms in my car 3Kms in a Yaris and my dads camry was spot on.

[bornwild]

Having said this, I would still go by the speed on the speedo. Reason is, if you get a ticket, a GPS device will not cover you in court if your going to dispute it. At least you can have your speedo tested by a mechanic to prove your defence.

That's wrong there mate :). Quite a few cases lately where people have defended themselves from fines using GPS'. Most famous one I can think of is the case that was presented on CA or TodayTonight couple months back.

GPS' are more accurate than your speedo, btw. Professionals use GPS' to measure quarter miles, 0-100 etc. without too much hassle.

[imay]

I believe my GPS gives me a more accurate speed reading than my speedo simply because it computes this information from several satellites which cross references my position to give an "true" speed reading . . . however, as stated, there are a number of reasons why your car speedo is actually giving a false reading. As long as the car speedo isn't reading faster than the GPS, follow the speedo's indicated speed and be happy with that. Less likely to get you into trouble!

[bornwild]

i get 4Kms in my car 3Kms in a Yaris and my dads camry was spot on.

Cars are usually doing anywhere from 96-98km/h on an indicated 100km/h....this is industry practice as someone said to keep you on the safe side, it's also a margin of error allowed for users changing the size of their rims.

[glider]

I believe my GPS gives me a more accurate speed reading than my speedo simply because it computes this information from several satellites which cross references my position to give an "true" speed reading . . .

mine is commonly connected to 8 satelites but can have up to 12 or 16 i believe

yeah basically the question just came down to me wondering whether my car's speedo needed adjusting... from the posts it seems that is not the case

[poita]

gps - speedo on calibra accurate up to 140km/hs
gps - speedo on my mazda 6, speedo bout 10% faster

[sooty]

Cars are usually doing anywhere from 96-98km/h on an indicated 100km/h....this is industry practice as someone said to keep you on the safe side, it's also a margin of error allowed for users changing the size of their rims.

i understand a margin for error, most cars have this etc...but what scared me was that at an indicated 100km/h a subaru (think it was an outback)...was doing 94km/h...that's a dangerous misrepresentation if you didn't know about it, cos you're either going to be going considerably slower than traffic, or consider yourself to be speeding...

[bornwild]

mine is commonly connected to 8 satelites but can have up to 12 or 16 i believe

yeah basically the question just came down to me wondering whether my car's speedo needed adjusting... from the posts it seems that is not the case

Nah you're on the safe side mate.

Another reason why your speedo reads out 100km/h at 96km/h is due to 60mph equalling 96.6km/h.

You'd have a problem only if you're speedo was actually reading lower than the GPS.

[bornwild]

i understand a margin for error, most cars have this etc...but what scared me was that at an indicated 100km/h a subaru (think it was an outback)...was doing 94km/h...that's a dangerous misrepresentation if you didn't know about it, cos you're either going to be going considerably slower than traffic, or consider yourself to be speeding...

Yeah well 94km/h is a bit too much of a difference. You'll find modern cars(our AHs) are sitting at about 98-99km/h at an indicated 100km/h.

And yeah outbacks would have off-road tyres, maybe the tyre pressure was too low etc. :)

[glider]

i understand a margin for error, most cars have this etc...but what scared me was that at an indicated 100km/h a subaru (think it was an outback)...was doing 94km/h...that's a dangerous misrepresentation if you didn't know about it, cos you're either going to be going considerably slower than traffic, or consider yourself to be speeding...


I always like to sit on speed limit on the motorways... always wondered why people were passing me (apart from the ones that were speeding) - i guess some cars just have less margin of error

[sooty]

Yeah well 94km/h is a bit too much of a difference. You'll find modern cars(our AHs) are sitting at about 98-99km/h at an indicated 100km/h.

And yeah outbacks would have off-road tyres, maybe the tyre pressure was too low etc. :)

mine's obviously not got factory wheels (225/40/R18 [same as SRi-T] makes my speedo apparently 0.8% too slow) but i sit at about 104-105 and through the speed checks on geelong road get bang on 100 :cool:

[bornwild]

Yours are 18"s, yeah?? I guess Opel would have incorporated that into the error thing as 18" are optional on all astras over there :)

[Dave]

Agreed with all the above, my speedo is 4-5k out acording to my gps, Ive gone through lots of speed cameras sitting 5k over by my speedo and dead on according to my gps....... i gues i trust it now :p

[Tigra]

That's wrong there mate :). Quite a few cases lately where people have defended themselves from fines using GPS'. Most famous one I can think of is the case that was presented on CA or TodayTonight couple months back.

GPS' are more accurate than your speedo, btw. Professionals use GPS' to measure quarter miles, 0-100 etc. without too much hassle.

Thanks for the feedback, will keep this one in mind.

[blueraven]

Yeah well 94km/h is a bit too much of a difference. You'll find modern cars(our AHs) are sitting at about 98-99km/h at an indicated 100km/h.

And yeah outbacks would have off-road tyres, maybe the tyre pressure was too low etc. :)

Nope, its a subrau thing, the speedo read 8-9% high on all of them.


You can however get the dealer to adjust it, which i will be doing on mine.

[Calibrated]

white cali is 8km out. 17's. Stock are 15's. Turbo cali is spot on all the way through to an undisclosed speed.

[Peety117]

GPS' are more accurate than your speedo, btw. Professionals use GPS' to measure quarter miles, 0-100 etc. without too much hassle.

The accuracy the professionals get would be archived a differential GPS set up, where you can get, sub-cm accuracy.

http://en.wikipedia.org/wiki/Differential_GPS


mine is commonly connected to 8 satellites but can have up to 12 or 16 i believe


There are only a maximum of 24 satellites working at any give time, 12 for each hemisphere, but its virtually impossible to pick up 12 satellites.

Standard GPS is precisely inaccurate.

http://en.wikipedia.org/wiki/Accuracy

I trust my GPS with the speeds it reads out +-2km/hr, given that i have a strong signal and the signal is not inhibited from trees and tall buildings (IE, Natio/City)