Indicated Speed vs Real Speed

[Vish]

Drove past a speed detectors that shows yours speed and it said I was doing 52 when I was doing an indicated 59/60, over 13% difference!

Is the speedo met to be that inaccurate or do I need to get my car recalibrated?

 

[kully]

normally that bad top end iirc , always though its about 10%

 

[Vish]

Yeah I thought it'd be around 10% out at around 80+mph, but not that big a difference at that speed. Is it possible to have it recalibrated so thats its only a few mph out all the time?

 

[ColinG]

i know the cruise and limiter functions dont correlate with the speedo


50mph on cruise shows about 55 in a v6

 

[kully]

have you had bigger wheels fitted , they puts the speedo out too .

 

[Vish]

Nope car is standard as are tyre sizes. Obviously, never notice it in everyday driving, so guess I'm not actually driving as fast I think I am, might be a licence saver lol unless the speed check I passed is incorrect? Maybe get my dad to try in his car. I'll try a lower speed tomo as its on my journey.

 

[MikeLYR27]

The cruise should be pretty accurate, had cruise and satnav the other day and there was only about 1mph difference.:S

 

[AlexS]

138 jelly babies indicated was 130 gps in my car lol.

 

[Vish]

Thats about 6% inaccuracy, which I'd be ok with at 130 (if my car went that fast) but 8mph out at 60 is a bit of a joke! Over 13%!

Will a decent garage have the right tools to make it a bit more accurate?

 

[Benbosh]

My cup was sat at 135 and gps indicated 133 so mine is pretty accurate

 

[Gary-Lew]

75 on my speedo is 70 on my Talex/TomTom

 

[Vish]

Sounds like mine is well out

Shall I just assume I'm doing aound 5mph slower than I think I am at lower speeds?

 

[D1CKY]

I have noticed this when i have my Road Angel in car.

The speedo overreads slightly at 70mph, it reads 66/67mph on Road Angel.

But i guess the speedo overread must get worse the faster you go

I've never really tried above 110 with Road Angel in car to check. .

So to get a really accurate top end speed, the Road Angel it has to be using a private Road of course

 

[D1CKY]

Drove past a speed detectors that shows yours speed and it said I was doing 52 when I was doing an indicated 59/60, over 13% difference!

Is the speedo met to be that inaccurate or do I need to get my car recalibrated?

By Speed Detectors :S

Do you mean the illuminated signs that display your speed as you approach them ?

 

[Vish]

I have noticed this when i have my Road Angel in car.

The speedo overreads slightly at 70mph, it reads 66/67mph on Road Angel.

But i guess the speedo overread must get worse the faster you go

I've never really tried above 110 with Road Angel in car to check. .

So to get a really accurate top end speed, the Road Angel it has to be using a private Road of course

Well I know the speedo i ment to be a lil overestimated, built in to all cars I beleive, getting more inaccurate the faster you go. Top end doesn't concern me cos If I'm going that fast, I can get caught by coppers anyway. Just more of a concern on A roads.

Drove past a speed detectors that shows yours speed and it said I was doing 52 when I was doing an indicated 59/60, over 13% difference!

Is the speedo met to be that inaccurate or do I need to get my car recalibrated?

By Speed Detectors :S

Do you mean the illuminated signs that display your speed as you approach them ?

Yeah it was a illuminated sign (don't know the right name). Must have been put up within the last couple days, on the a main road but can easily hit double the limit of 40....so I heard.

Just shocked at the difference!

 

[Rob's172]

138 jelly babies indicated was 130 gps in my car lol.

i'm sure ive seen 150 chocolate buttons, no chance i tell you

its a bit like the bhp figures as well:dapprove:

 

[Phoenix]

also dont forget the speed detectors are in accurate by a very maximum of 8mph!!!!

 

[Vish]

also dont forget the speed detectors are in accurate by a very maximum of 8mph!!!!

Really??? Are they not ment to be very accurate? Thought they used same technology as police radar guns...

 

[GordonD]

also dont forget the speed detectors are in accurate by a very maximum of 8mph!!!!

Really??? Are they not ment to be very accurate? Thought they used same technology as police radar guns...

Those sorts of devices usually use sensors in road not radar or laser.

 

[GordonD]

Drove past a speed detectors that shows yours speed and it said I was doing 52 when I was doing an indicated 59/60, over 13% difference!

Is the speedo met to be that inaccurate or do I need to get my car recalibrated?

Do not rely on it being out by that percentage at all speeds. I know from GPS that when mine reads 60 km/h its only doing 54 (10% out), but that only increases to 8 km/h out at 110, then it stays at about 8-10 km/h from there on up.

 

[Vish]

Oh right. Thanks Gordon. I'm going to try and drive past it at 40mph and see what it says today.

 

[Ex-User (19928)]

Those things (if it's what I think you mean, i.e. those signs that are put up for a day that show your speed and a smiley face if you're under the limit) use radar technology.

Radar speed guns etc etc are quite inaccurate themselves, they take a few seconds to get a reading, in which time any change in speed can again cause bigger error.

There are many cases of people doing independant tests and having a radar gun show a massive error, the biggest difference I remember off hand was 33mph.

GPS isn't amazingly accurate either, unless you maintain a constant speed and just take a load of averages. GPS in car sat navs are only accurate to about 10m or something silly like that.

 

[Tom H]

On aston martins test track we managed about 140 indicated and that showed up as 135 on the tomtom

Then the pace car wacked his brakes on and it scaring braking hard from those speeds!

 

[Studio 182]

yeah i wouldnt think them roadside speedos are that accurate TBH

 

[Vish]

Basically its an illuminated sign put up on a lamp post which just shows your speed.

Went through at around 47mph on the speedo and it came up as 44 on the sign. So maybe nothing wrong with my car then.

Shouldn't radar guns be dead accurate if they're going to convict ppl of speeding?

 

[D1CKY]

Shouldn't radar guns be dead accurate if they're going to convict ppl of speeding?

I agree with you there.

There was a thing on tv a while ago about Police speed Guns etc being insufficient at catching people, but i can only remember a few things they went over.

I think they said when coppers are using them they need a really steady hand as any movement can give wrong speed i think :S

As well as using them in police car that can do something to readings, i don't know what though :S

 

[Vish]

I don't like the sound of that. So only fixed cameras are reliable then? How on earth are you able to prove you weren't speeding if its not accurate and you KNOW you definitely weren't speeding?