speedometer or GPS?

[buttercup]

Is it the same on your car, that the GPS speed reading is significantly lower than the speedo reading? I checked it on my car, and at a steady 100kph on the speedo, the GPS indicated I was actually doing 93.

I just changed to winter tires, and it was exactly the same, so it's probably not due to the tires being slightly out of spec.

Do car manufacturers deliberately make our speedos read way above the speed we are actually doing?

I take it that the GPS is the more accurate reading. Is it possible the GPS speed could be wrong?

 

[blackrock13]

Is it the same on your car, that the GPS speed reading is significantly lower than the speedo reading? I checked it on my car, and at a steady 100kph on the speedo, the GPS indicated I was actually doing 93.

I just changed to winter tires, and it was exactly the same, so it's probably not due to the tires being slightly out of spec.

Do car manufacturers deliberately make our speedos read way above the speed we are actually doing?

I take it that the GPS is the more accurate reading. Is it possible the GPS speed could be wrong?

The accuracy of the GPS speedometer might be affected by the length of trip your sampling. I would take the results from the cars speedometer over the GPS reading.

 

[pc0mo]

there is a slight delay of the Gps and your car , just remember that a Gps is give or take 100 m but for most things its spot on due to the triangulate and positioning of multiple satellites . so that 93 could actually be a 97 or even a 100 its quite possible ,

 

[Master_Bates]

the reading on my gps seems to be quite consistent with my speedometer when i'm maintaining my speed. It only seems to lag when accelerating/decelerating.

 

[papasmerf]

my Tom Tom seems to be spot on for speed but lags a bit on compass position.

Best way to check speed is to use mile markers and a stop watch.
This way you can do the math and verify your cars speedometer.

 

[oldjones]

Most speedos read several kph higher than actual, and have for decades, if not since the day they were invented. Tends to make the car seem a better performer (and saves you from the odd ticket) which is why manufacturers do it.

 

[simon482]

are you driving on a flat road or road with hills ?

also is the sun shining or was it cloudy. clouds can slow down a gps signal.

 

[Old Milwaukee]

The gps is far more accurate, period. It may not be perfect, but over a straight, consistent speed and distance, it's almost spot on. Speedo's are not reliable, mainly for legal reasons. What if it were discovered that someone in a serious crash could prove that the speedo read 100 kph, and the driver was actually going say 110 kph? Manufactures error on the safe side, strictly for legal reasons. Do not trust you cars speedo, some are way off!

 

[simon482]

The gps is far more accurate, period. It may not be perfect, but over a straight, consistent speed and distance, it's almost spot on. Speedo's are not reliable, mainly for legal reasons. What if it were discovered that someone in a serious crash could prove that the speedo read 100 kph, and the driver was actually going say 110 kph? Manufactures error on the safe side, strictly for legal reasons. Do not trust you cars speedo, some are way off!

i am just gonna assume you are kidding and go about my day.

 

[blackrock13]

i am just gonna assume you are kidding and go about my day.

I certainly hope he was. I could be wrong, but that would mean that every speeding ticket could be challenged and grind traffic court to a stand still. Odometers maybe, but not speedometers.

 

[simon482]

I certainly hope he was. I could be wrong, but that would mean that every speeding ticket could be challenged and grind traffic court to a stand still. Odometers maybe, but not speedometers.

gps told a woman to turn into a lake, how accurate could it really be.

 

[papasmerf]

gps told a woman to turn into a lake, how accurate could it really be.

How smart was she for thinking it must be a big puddle and drove into it?

 

[simon482]

How smart was she for thinking it must be a big puddle and drove into it?

she was a moron there is no question of it. to think that a gps is more accurate then the functions built into your car is just as dumb.

 

[blackrock13]

she was a moron there is no question of it. to think that a gps is more accurate then the functions built into your car is just as dumb.

i don't expect perfection in maps and the like. I've caught about 3 errors in Perly's in the past few years.

 

[simon482]

i don't expect perfection in maps and the like. I've caught about 3 errors in Perly's in the past few years.

perly's ever tell you to drive into a pond or off a cliff ? would you stare at the perly's or the road ?

 

[simon482]

i don't expect perfection in maps and the like. I've caught about 3 errors in Perly's in the past few years.

and no map or gps or anything of the sort is perfect.

 

[JohnHenry]

Throw in a little arithmetic.
Speedometers measure tire revolutions per unit of time. The problem comes in determining distance traveled per tire revolution. Typical car tires are 28 inches in diameter new, but with 1/4 inch of tread worn off are now 27.5 inches in diameter; a 2 percent variation.
Tire pressure will vary the rolling radius of the tire. Tires when mounted on the car are no longer round. The actual radius is the distance from the center of the axle to the ground.

 

[oldjones]

I certainly hope he was. I could be wrong, but that would mean that every speeding ticket could be challenged and grind traffic court to a stand still. Odometers maybe, but not speedometers.

No he's not wrong, which is one reason why JP's want more evidence fom you than. "My speedometer said I was under thirty". They've never been accurate. Though they always overstate the speed, we cannot say the same about the guy looking at them. Another reason your speedometer's not evidence.

 

[oldjones]

according to the law it can't be more that +/- 3 km/h off

According to UN convention the error must never understate the actual speed of the car and may not be more than 10%. Whose law are you referring to asn?

And as John Henry points out below, since tire size—due to inflation or what you decided to buy—affects the reading, it's obvious speedometers are only a guide, never to be trusted as your true speed.

But we knew all that. The real question is: How is the GPS determining speed, and how accurate is that?

 

[raydeon]

there is a slight delay of the Gps and your car , just remember that a Gps is give or take 100 m but for most things its spot on due to the triangulate and positioning of multiple satellites . so that 93 could actually be a 97 or even a 100 its quite possible ,

Since 2000 the civilian GPS accuracy has been increased to 20 meters (When then US President Bill Clinton ordered the military to turn off Selective Availability). Some GPS manufacturers even claim 15 meters accuracy.