How accurate/inaccurate is your speedo?

Bored-Stupid

Howdy?

Just wondering how accurate your speedo is?Can remember seeing an old Top Gear (with Quentin Wilson reporting) on how inaccurate speedos are...like up until 50mph your going faster than your speedo and above that your going slower but I cant remember the exact figures!

Also how fast can you go on the motorway taking into account the inaccuracies in your speedo and the leniency of the Gardai?

peasant

Every car is different.
Also tyre size affects the readout.

Get yourself (a loan of) a GPS and find out exactly by how much your speedo is out.

Stephen

According to my GPS, mine overreads by 6kph at most speeds. '07 Honda Civic.

Thomas_S_Hunterson

I think most speedos will over-read by a certain amount.

Kimbot

All speedos have to overread by at least 3-5% i think it is.

unkel

It's a legal requirement that speedos overread by a minimum of 0% and a maximum of 10%

As peasant said, get a GPS to check your real speed. If your tyres are not the manufacturer's default tyres, use a tyre calculator to check the impact on the indicated speed

TychoCaine

My Volvo S80's speedo reads 80mph (128kph) when the GPS says I'm doing 120kph. That's a 6.7% over-read. I'm using virtually new factory spec tyre sizes.

Cars with seriously out of whack speedos are probably the cause of my pet-hate: people who drive at < 90kph on a main road. :mad: As far as they're concerned they're driving at or near the speed limit, so they shouldn't have to pull in to let an obvious "speeder" past. In fact they're doing the gardaí a favour by preventing speeding. But pretty soon you end up with a convoy of 5+ cars that is unsafe to overtake on what passes for a main road in this country.

It doesn't bother me all that much on a short journey, but it can make my weekly Dublin - Shannon drive pretty tedious sometimes. The N7 from the end of the motorway as far as Nenagh is 70km of single carriageway road that has a solid stream of traffic in both directions in the evenings. Spots to safely overtake a convoy like that safely are rare.

JoeySully

Yeha iv wondered this too as iv passed two guards with guns, doin 120 in a 100 zone and never got flashy lights behind me, but ppl tell me that they will only lookin for the real speeders. ie 20+ kph over the limit. more rumours iv heard is that the guards give a ~10 leway in speed ie if the limit is 100 and they read 110 then your ok but i think this is down the the guard in question.

Bored-Stupid

Well I passed the camera van todau on the motorway (the red transit with the blacked out rear windows) sitting on top of a fly over,was doing 128kph (80mph) in the 120kph motorway limit?Will I expect a lovely letter in the post?
Also my car is English reg and I havent brought it in yet!

peasant

How would they send you a letter when the car isn't even registered here?

BrianD3

TychoCaine wrote:

Cars with seriously out of whack speedos are probably the cause of my pet-hate: people who drive at < 90kph on a main road. :mad:

Ther must be an awful lot of crap speedos out there then

Personally I think that peope driving at about 90 km/h is more likely to be because they think the limit is still 55 mph :rolleyes:

templetonpeck

OMG!! I was only thinking yesterday how great my car is. Depending on the amount of pressure you put on the accelerator the speedometer responds almost immediately and so accurately, making it an absolute pleasure to drive. I had a punto before and your foot'd be up and down the accelerator tryin to get it exactly on the speed limit. It never occurred to me that the damn thing could be inaccurate???!!

Anyone know about Honda Jazzes???? Are they known for their inaccuracy??

It BeeMee

TychoCaine wrote:

My Volvo S80's speedo reads 80mph (128kph) when the GPS says I'm doing 120kph. That's a 6.7% over-read. I'm using virtually new factory spec tyre sizes.

Just as a matter of interest, have you checked it at other speeds?
I read a comment somewhere here recently that the margin changes ie higher margin at slower speeds or whatever. Just curious if the 6.7% is constant across the speedo.

TychoCaine wrote:

Cars with seriously out of whack speedos are probably the cause of my pet-hate: people who drive at < 90kph on a main road. :mad: As far as they're concerned they're driving at or near the speed limit, so they shouldn't have to pull in to let an obvious "speeder" past.

Okay, 90 is a bit low, but anyone that appears to be driving between 90 and 100 are driving according to the best information available to them.

No everyone assumes their speedo is wrong, they can't be faulted for that.

Standard Toaster

How acurate would the GPS unit be?
My speedo is out by 7mph so so.....17" on 01 Astra

Stephen

GPS's are pretty much dead on, but you have to take a second or two of latency into account due to the massive distance away that the satellites are, so just travel at a constant speed to get a good measure of it from your gps.

DonJose

I drive an 06 Golf, when on the motorways the speedo reads 120kph, but my sat nav reads 110kph. I just came back from the States, I rented a Chevrolet SUV and the speedo was dead on accurate, had the exact same reading as the sat nav.

unkel

It BeeMee wrote:

I read a comment somewhere here recently that the margin changes ie higher margin at slower speeds or whatever. Just curious if the 6.7% is constant across the speedo

In most cars, the difference between indicated and real speed increases with speed. Most speedos would be pretty accurate at 50km/h, but a lot less accurate at 120km/h

TychoCaine

unkel wrote:

In most cars, the difference between indicated and real speed increases with speed...

...which is why I quoted a percentage, and to answer "it be me", the % gap appears constant. For example, @ 60kph on the GPS, the speedo reads dead on 40mpg, which again is a 6.67% over-read.

Gary

jayok

templetonpeck wrote:

Anyone know about Honda Jazzes???? Are they known for their inaccuracy??

Honda's are fairly consistent with their "inaccuracies" across models. It used to be 3-4 mph. So I'd say it's about 5-6km/hr over. i.e. at 100 kph you're actually doing 94-95 kph.

<edit> See Stephen's reply

It BeeMee

TychoCaine wrote:

...which is why I quoted a percentage, and to answer "it be me", the % gap appears constant. For example, @ 60kph on the GPS, the speedo reads dead on 40mpg, which again is a 6.67% over-read.

I guess it varies from manufacturer to manufacturer?

I err on the side of caution, and assume 5% on my car. Well I say "assume", I kinda proved it once by driving a set distance at a fixed speed and timing it.

The joys of cruise control.....

Seth Brundle

jonny24ie wrote:

All speedos have to overread by at least 3-5% i think it is.

unkel wrote:

It's a legal requirement that speedos overread by a minimum of 0% and a maximum of 10%

a pint (via paypal money) for the first person who can show me the legislation

lafors

Stephen wrote:

According to my GPS, mine overreads by 6kph at most speeds. '07 Honda Civic.

Snap, same car (cept 06)
6km/h over.

faceman

kbannon wrote:

a pint (via paypal money) for the first person who can show me the legislation

its part of the S.I. No. 305/1978: European Communities (Motor Vehicles Type Approval) Regulations, 1978.

unkel

kbannon wrote:

a pint (via paypal money) for the first person who can show me the legislation

Slàinte so

I've shown it here before, but can't find my post. The legislation is here. See 2.3.7. In my first post on this thread I forgot to mention that it is a maximum of +10% + 4km/h over the actual speed

Alun

kbannon wrote:

a pint (via paypal money) for the first person who can show me the legislation

It's been posted here on the Motors forum by, amongst others, my good self, many times before. Surprised you hadn't noticed it, being a mod and all Anyway, maybe it's a candidate for a sticky seeing as it crops up so often.

faceman

just give us all the beer money kbannon and be quick about it!

Seth Brundle

PM me your paypal addy!
I don't recall seeing the legislation before - I just recall people saying that it was law.

faceman

why not give the dosh to charity instead?

cianclarke

Interesting thread!
My 2000 1.6 passat's speedo is close to bang on. At 40mph it's doing 38mph.
At 60mph gps said 57/58mph.
At 65mph gps said 62mph.

Breizhoneg

Hi there

having a strange experience with the speedo on a 97 fiat punto

up to 40 km/h everything checks out

when going over 50 the speedo will show 50 but would actually be doing around 60 km/h and the difference increases as soon as you accelerate.

like if your speedo shows 60 km/h you are almost driving at 75 km/h

Any idea on how to fix this ?

Cheers
Franck

CiniO

TychoCaine wrote: »

My Volvo S80's speedo reads 80mph (128kph) when the GPS says I'm doing 120kph. That's a 6.7% over-read.

Exactly the same story with my car Mazda 6.
Over-read by 6.7%, and this rule works at all speeds.

It's actually very handy, as I can very easily convert MPH speed shown by speedo to km/h values in my head.
Just multiply by 1.5


f.e. I know
20 is 30.
40 is 60.
50 is 75.
80 is 120.
100 is 150.
140 is 210.

Obviously speedometer has scale in km/h but font is tiny, and it's much easier for me to quickly calculate the above, than focus to read the tiny font.

pippip

ZOMBIE THREAD!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!