Speed difference between GPS and speedo

[rkbharat]

I am finding a difference of 5KM/H between my speedo and the GPS log. GPS showa 5km/h less than what speedo shows.

Which is more accurate? I think GPS should be more accurate.

[spadival]

I would say GPS is more accurate, but then people say it all depends on what chipset is used on the GPS device and the refresh rate - 5hz versus 1hz.

But when you are moving at a steady speed (by checking the speedo) without accelerating, it should rule out refresh rate induced delay if any .

[Blue Thunder]

It is a known fact that most speedometers have an error on the positive side : they always show a speed which is slightly higher that the "actual " speed of the vehicle. You can verify this by checking the auto-reviews done by expert testers, who use sat-based instruments (more complicated form of GPS) to do the speed testing.

(some road test results I remember : Scorpio had an odo reading 160kmph, when it was actually doing around 153kmph; Mercedes doing 187kmph actual on a speedo read of 190kmph)

So, your GPS reading is probably giving the more accurate speed.

[tsk1979]

If you are not accelerating, GPS will give you accurate speed

[rkbharat]

Tanveer, even if I am accelerating or de-accelerating, the refresh rate is pretty fast ans GPS maintains the speed in milisec time delay.

Screen shot of trip meters attached:

Originally Posted by tsk1979

If you are not accelerating, GPS will give you accurate speed

[bblost]

I know that the Speedo gets influenced by change in tire size.

Does GPS get influenced by road gradient?

[tsk1979]

Its not the refresh rate, but the error. Normally a GPS will show the error. It can be as low as 15 meters and as bad as 200 meters if reception is bad.
In such conditions when you accelerate, the speed can fluctuate wildly. However in my GPS the maximum speed marker is intelligent enough to smooth out the fluctuations.

[rkbharat]

Gradient will not really affect the speed as such but it may affect total distance covered, if software you are using is not written properly. Because GPS chip do logs the change in altitude, which need to factored in while calculating the change in position. Like while driving on hills.

Speed will be affected only in case very steep gradient.

Originally Posted by bblost

I know that the Speedo gets influenced by change in tire size.

Does GPS get influenced by road gradient?

[rkbharat]

Yes, you are right. error will be high in case of bad weather, driving in Hills, or High Tower city. But it will be mostly accurate on highways with clear weather. Also depends on the GPS receiver.

Originally Posted by tsk1979

Its not the refresh rate, but the error. Normally a GPS will show the error. It can be as low as 15 meters and as bad as 200 meters if reception is bad.
In such conditions when you accelerate, the speed can fluctuate wildly. However in my GPS the maximum speed marker is intelligent enough to smooth out the fluctuations.

[Maverick1977]

GPS is & shall always be more accurate than your car speedo. Car speedos are generally set with a marginal error by the manufacturer, so that a driver gets a feeling of going much faster than he/she is actually.
The accuracy of a speedo or rather the lack of it will be clearly experienced when one applies sudden (not emergency) breaks. The Speedo needles will show 30 kmph while the car is almost coming to a stand still.