Actual Speed

[mackt 1]

Hi

Just wondering about the accuracy of your car speedo vs a GPS unit.

Does anyone have any idea how accurate the GPS indicated speed is.

If I have cruise set on 110 - GPS says 100km/hr (am I playing with possible demerits)?

[Nobimmer 694]

Pretty sure your GPS will be wrong.

[dubman 39]

if ur tyre size and rims match factory specs the speedo would be right. if not, the speedo would still be more accurate than the GPS.

[greenday-rulz21 6]

I don't really have much experience in this but I'd tend to disagree.

My GPS, according to Navman, is accurate to 4 meters. Which is nothing really.

Cruising at 110km/h on the speedo and the gps would say 102km/hish from memory. About 7km/h off i've figured.

[bravo 35]

Sorry Ashkan/dubman, but GPS (even low accuracy in-car units) generally provide a fairly accurate determination of velocity. Absolute positions might be ±15m, but relative positions a second or so apart are usually much more accurate as they are subject to nearly identical error, and thus cancel out.

It depends on whether your unit uses position or the doppler effect to determine speed, but most modern units use position which is fairly good.

I'd trust the GPS with a margin of error of a couple of percent versus 10%+ for your car speedo.

[mackt 1]

Both my Garmin etrex and TomTom One give the same speed with 7 satellites.

The Garmin is accurate when I am on my MTB i.e MTB speedo matches GPS speed??

[kiwi535 538]

sppeedos are ussually optomistic by a good margin,the theory is the manufacturers cannot be accused of having speedo that reads UNDER the speed you are actually doing.Perhaps its all a plot to get us all driving slower.

[kingkarl 136]

At an indicated 100km/h my car is doing 91km/h on the GPS.

I believe it, because when I jump in my parents late® model BMWs the GPS reads about 98km/h at an indicated 100. That and if I sit at an indicated 100 I'm way slower than most of the other open road traffic, but I keep up at about 110.

[dubman 39]

i take my point back about speedo's, GPS' are more accurate after some googling, but in terms of demerit points and speeding tickets, wouldnt it be easier to defend yourself if you used the factory speedo reading as your defense?

[kiwi535 538]

i take my point back about speedo's, GPS' are more accurate after some googling, but in terms of demerit points and speeding tickets, wouldnt it be easier to defend yourself if you used the factory speedo reading as your defense?

if you do the posted sped limit accoroding to your speedo(assuming coreect wheel and tyre size) you will NEVER get a speeding ticket because you will a lways be doing less than the actual speed your speedo is saying

[1rotty 40]

I always understood most odo's show 5-7ks approx under actual speed as a safety feature.

Mine all have about this discrepancy versus my gps.

Cruise set at 112 lol.

[Wom 0]

I wonder how accurate those speed display signs are

Using a passenger and one of those signs, I discovered that my speedo is only 2kph above what I'm really doing

I think they get a bit confused when there's lots of traffic, so I tried it when it was the middle of the night with no other traffic around

[Eagle 1662]

My GPS is dead accurate, goes up by 1km when you tap to cruise control forward which tell it to increase speed by 1km. All my E34\E32 have read 4-5km lower than speedo but my E30 reads 10km low.

Speed signs seem read a coupe of kph higher than gps for me

[pureboiracer 0]

those speed display signs arnt accurate at all. around wellington they vary about 7kmh between different ones. your car speedo is usually a max of 10% out from factory, gps i find are within 3 or 4% i drive past speed cameras at what says 117kmh in my car and havnt got a ticket yet.

[Forrest 35]

There is a good article about all this in consumer magazine this month

[Apex 693]

All cars are 7-10kph out on the speedometers, it’s a safety feature. I thought this was common knowledge.

If you can access yours cars dealer menu you will see the real vehicle speed displayed against the speedometers.

[antil33t 90]

Only problem I found with GPS and setting it to your cruise control... is other people are doing an indicated 100kph so you have to slow down all the time D:

[Lucan 196]

All cars are 7-10kph out on the speedometers, it’s a safety feature. I thought this was common knowledge.

This.

I always drive just under 110 on open roads.

The Navman doesn't signal I'm exceeding the 100 limit till I hit around 115 though

[Wom 0]

All cars are 7-10kph out on the speedometers, it’s a safety feature. I thought this was common knowledge.

I always thought it was only normally 2 - 4kph out for cars and something like 8 - 10kph for motorbikes

[greenday-rulz21 6]

Newer cars are usually around 3-4km out. Older cars tend to be from 7 to even 15km out. E30 speedos are notoriously bad.

[nastnas 9]

does GPS take into account elevation? like if you were going up an incline of 20 degrees.... does that factor into the GPS unit and adjust your speed shown on the GPS unit?

Im sure all of you know what i mean....

[huff3r 347]

does GPS take into account elevation? like if you were going up an incline of 20 degrees.... does that factor into the GPS unit and adjust your speed shown on the GPS unit?

Im sure all of you know what i mean....

They take a lock from several sattelites, so im sure it does. It isnt simple 3-way triangulation, i believe they prefer to have 9? or some other number to lock onto haha.

[nastnas 9]

thats pretty cool...

[Apex 693]

Newer cars are usually around 3-4km out. Older cars tend to be from 7 to even 15km out. E30 speedos are notoriously bad.

Yea, that is correct, some magazines test for speedometer error and publish it in their stats list.

I generally fall victim to the cruise control set at 110 trap, drives me nuts. I find backroads are the only way to travel in NZ if you want to maintain speed..

[mackt 1]

Hi

Been doing a bit of research - looks like GPS is the most accurate, even if you take into account corners, ascents and descents - hopefully will find something from a manufacturer to support this soon!

http://www.caranddriver.com/features/02q2/...al!-feature

Edited April 20, 2010 by mackt