Speedodometer accuracy - myth or fact?!!

dazzerb · 19-04-2009 11:52PM #1

Always heard that speedometers are not accurate so if speed limit is 100 you can safely go 105-110 and the guards highly accurate equipment will show you are driving legally.

Now this maybe makes sense on old cars where everything is analog and I can see where the speedometer is not exactly linear but on modern cars with electronic trip computers surely the car folks have cracked this nut and with most cars these days you are going the speed you think you are going.

Also if the speedometer is inaccurate then would your odometer be inaccurate too?!!

can we bust this myth?

Anan1 · 19-04-2009 11:54PM

Depends on the car, many new cars still overread by a significant %.

qz · 20-04-2009 12:00AM

I've been searching for an opportunity, sorry.

Esel · 20-04-2009 03:00AM

Too many variables to generalise. Speedo accuracy will be affected by such things as wheel/tyre size and tyre wear. If your speedo is inaccurate, your odometer will be also.

My car speedo is perfectly matched with satnav indicated speed.

.Longshanks. · 20-04-2009 06:03AM

My 08 corsa can be set to also display a digital readout of speed. When traveling at 120kmph on the dials both the digital readout and the satnav both tell me I'm going 114-115kmph

Padraig Mor · 20-04-2009 07:55AM

My speedo overreads between 2 and 3%.

Cyrus · 20-04-2009 08:27AM

dazzerb wrote: »

Always heard that speedometers are not accurate so if speed limit is 100 you can safely go 105-110 and the guards highly accurate equipment will show you are driving legally.

Now this maybe makes sense on old cars where everything is analog and I can see where the speedometer is not exactly linear but on modern cars with electronic trip computers surely the car folks have cracked this nut and with most cars these days you are going the speed you think you are going.

Also if the speedometer is inaccurate then would your odometer be inaccurate too?!!

can we bust this myth?

a sat nav will show a truer reflection of your speed, my last 4 cars have all show about 5-8% higher on the speedo compared to the sat nav

Stephen · 20-04-2009 08:29AM

My Honda (digital speedo) reads 106kph when GPS indicates actual speed to be 100kph. Its a bit closer at 50kph.

BlackWizard · 20-04-2009 08:31AM

I think Im up near 8% on mine. I only checked this last night when I wasn't paying attention to my speed and I passed some big ass speeding cameras on the M1. At about 140 I was doing about 130. Those couple of % makes a big difference when going fast. So me going 120 means I'm going about 110 in the Fast Lane.

Matt Simis · 20-04-2009 09:08AM

Thats the problem though, some people think they are "doing the limit" in the "fast lane" and refuse to move out of the way.

BlackWizard · 20-04-2009 09:13AM

I drive in the fast lane because I don't want to go into the slow lane. Too many trucks and I might have to move into the fast lane to eventually anyway to overtake someone.

(Over taking lane is for over taking..grr we need a weekly Fast Lane/Over taking lane thread so we can dump frustration into it ever day. Today fecking HGVs where hogging it

)

Totally off topic discussion though! I don't think it's such a bad idea for me having the speedo that little bit over. I often find myself going above 120kph on the motorways so it's a nice buffer incase I do happen to run by a speed camera while going over the limit.

Jip · 20-04-2009 10:07AM

I had a 07 Vectra over the weekend and 120k on the satnav was over 130 on the speedo. More or less the same for my 01 Octavia. Of course the slower you go the difference is less obvious but it's not just an issue in older cars.

20-04-2009 10:09AM

I am actually freaked out a little as I was going to start this thread today myself!

I was doing 60kph this morning and there is one of those axa speed checks on the road which said I was doing 56.

BlackWizard · 20-04-2009 10:12AM

I guess the moral of the story is, we should all driver faster.

rgunning · 20-04-2009 10:21AM

If you want to test it out, best to do what I did this morning and go speeding past two guards with a radar detector on the M7. They will drive up to you and show you what speed you were doing, and you can cross reference it with your speedo.

The service only costs €80 and you get loyalty points.

....

In fairness there were very nice about it, and I really deserved it. Didn't even try to use how late I was for work as an excuse.

unkel · 20-04-2009 10:29AM

Speedos legally can show any speed between the actual speed and the actual speed + 10% + 4km/h. So the speedo of a car doing an actual 160km/h can show anything between 160km/h and 180kmh (160+16+4).

We've done this many times before here, do a search

BlackWizard · 20-04-2009 11:56AM

unkel wrote: »

We've done this many times before here, do a search

Just because you've danced to the song before doesn't mean you can't dance to it again!

*does a dance*

L-M · 20-04-2009 11:56AM

Was in a Golf the other day, Golf read 100kms, Sat Nav read 96kms.

Anan1 · 20-04-2009 12:00PM

AFAIK, sat nav speed readings will only be accurate on a straight road.

Jip · 20-04-2009 12:51PM

The readings I took where on the M1 so I was going fairly straight.

.Longshanks. · 20-04-2009 01:12PM

Longshanks100 wrote: »

My 08 corsa can be set to also display a digital readout of speed. When traveling at 120kmph on the dials both the digital readout and the satnav both tell me I'm going 114-115kmph

My car

If anybody cares the 3132 figure is the digital revs!

maggy_thatcher · 20-04-2009 01:20PM

Longshanks100 wrote: »

My car

If anybody cares the 3132 figure is the digital revs!

I hope it really was foggy when you took that picture

.Longshanks. · 20-04-2009 01:27PM

maggy_thatcher wrote: »

I hope it really was foggy when you took that picture

I also hope i wasn't driving in the outside land of the motorway hoggin the "fast" lane at 3 o'clock in the afternoon taking a pic on my phone

Edit;
I was actually during the day and there was no fog. However it was a really bright day and the sun was really low and directly behind me. I turn my fog lights on at times like this so that drivers ahead / approaching can see me better.

L-M · 20-04-2009 09:32PM

Longshanks100 wrote: »

Edit;
I was actually during the day and there was no fog. However it was a really bright day and the sun was really low and directly behind me. I turn my fog lights on at times like this so that drivers ahead / approaching can see me better.

Lol, very good respone. What a way to get out of it. Can't fault it haha:P

Fishtits · 20-04-2009 09:38PM

Satnav or other GPS device will only record accurate speed when the receiver is traveling in a parallel plane to the satellite.

In common language, they're not accurate. Myth busted.

BTW, a good site to get an understanding on how GPS works is trimble.com, follow the tutorial.

Mr. Frost · 20-04-2009 09:40PM

Matt Simis wrote: »

Thats the problem though, some people think they are "doing the limit" in the "fast lane" and refuse to move out of the way.

They should be moving out of the way regardless.

P.C. · 20-04-2009 09:56PM

Longshanks100 wrote: »

I also hope i wasn't driving in the outside land of the motorway hoggin the "fast" lane at 3 o'clock in the afternoon taking a pic on my phone

Edit;
I was actually during the day and there was no fog. However it was a really bright day and the sun was really low and directly behind me. I turn my fog lights on at times like this so that drivers ahead / approaching can see me better.

.Longshanks. · 20-04-2009 10:04PM

Grow up. This thread is about speedodometers - start your own thread if you want to moan about fog lights

Anan1 · 20-04-2009 11:03PM

Longshanks100 wrote: »

Grow up. This thread is about speedodometers - start your own thread if you want to moan about fog lights

Perhaps you should grow up - the sidelights/fogs thing is for kiddies in Micras.;)

L-M · 20-04-2009 11:09PM

Charter wrote:

* No "why do people not turn off their fog light" threads - they have been done before.

The above sounds good, lol.

Yakult · 20-04-2009 11:18PM

Limerick man wrote: »

The above sounds good, lol.

ya, honestly if I were to see one more argument about foglights I think I will just give up on life or something :pac:

Speedodometer accuracy - myth or fact?!!

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