Speed vs. Speedo

[newkiebrown]

Does anyone have the same issue as me whereby when I pass the speed signs that tell you what you're doing it says I am doing 30 but I am doing 34 on my speedo?

It's been like that since I owned it and it has stock wheels on it 19" but wondered if anyone else has noticed this, same at 40 doing 44 on the speedo.

Just curious more than anything else

 

[Verminator]

Most speedometer have a 10% OFF the actual speed... Eg. 30 maybe 33,
40=44 this is in mph

 

[newkiebrown]

yeah MPH, I just wondered if it was normal and I'd just never noticed it. Just thought it unusual that 30 is 34 and 40 is 44 so it's 4MPH for both. I have never been able to try and 50 or more to see if it remains at 4MPH or, as you say, it's 10%

 

[Verminator]

Get a speed app on your phone and set it up and turn on when driving and compare.. But always use hand free and drive safe

 

[2retro]

Car speedo's are never right. That's why speed cameras give that 10% leeway (77 mph in a 70, etc.).

 

[stewartwillsher]

Our three "road" vehicles over-read (or is that under-read), according to the sat-nav (was Tom-Tom, now Garmin).
The cruisers (508 and RCZ) can be at about 125 to match the sat-nav 120 (both kph).
Useful, here, when encountering a sped camera on the Autovia.

 

[RCZ-R]

As far as I recall the speedo on my R was precisely +4 kmh over true GPS speed all across the range. No matter if 50 or 250 on the speedo, it always was +4 over GPS.
This made it easy to fly under radar and still maintain proper pace.

 

[Pedro]

A UK police force sent their brand new unmarked fast response car back after its speed calibration was incorrect the manufacturer said its because the actual speed of the vehicle is offset due to allowing for wear & tear on the ABS wheel speed sensors tyre wear etc car makers are apparently allowed to do this under European legislation therefore should you ever trust the speedo in even a new car?

 

[LeeS69]

It's not to do with ABS sensors (they are digital anyway) or tyre wear (though that will have an impact in the opposite direction - there can actually be significant difference in circumference between tyre brands - I learnt that a very very expensive way).

Warning - severe nerdism ahead

It's due to EU legislation - manufactures are allowed to let speedos over read but are absolutely not allowed to let them under read. So they generally all over read, only car I ever had that didn't noticeably do it was an old 911 (built long before this legislation!)

Here's the specific bit of legislation

5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).

0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

Gives you an allowance up to 7km/h of error at 30kph!!

https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:120:0040:0048:EN:PDF

 

[Pedro]

What's this then?

 

[RCZ-R]

Instead of installing calibrated gauges, the police should have purchased few mid 2000s Citröens C4 and C5. Or some Peugeot 508. The speedos in those cars showed the exact GPS speed all the time across full range.

 

[LeeS69]

What's this then?

Totally irrelevant to "abs sensors" and "tyre wear" is what that is... And actually confirms the regulations? Police might fit calibrated speedos... Not sure what your point is??

 

[LeeS69]

Instead of installing calibrated gauges, the police should have purchased few mid 2000s Citröens C4 and C5. Or some Peugeot 508. The speedos in those cars showed the exact GPS speed all the time across full range.

My 508 reads about 2-3mph above the actual speed at motorway speeds - compared against 2 GPS based units

 

[Pedro]

This must mean every car on the road is technically speeding I know they are very strict in the police fleet garages because one of my son's is working in one

 

[LeeS69]

This must mean every car on the road is technically speeding I know they are very strict in the police fleet garages because one of my son's is working in one

Erm, OK? not sure how the hell you're managing to work that out....!!??

Normal car speedos read over a little bit, because they aren't allowed to read under at all...

There you go, it's really simple....

It's not rocket science or a conspiracy....

Baffled what you find confusing??!!??

I set my "50" zone limiter on the 508 at 52, because that's actually 50... My motorway limit is set at 70 because I never ever speed but if I did it would be set at 79/80 because that would actually be 76/77 and below the normal prosecution threshold, but as I say I never go over 70....

 

[Pedro]

You've shot yourself in the foot twice I have explained the logic so what don't you understand & were not discussing 508's on this forum??!!!!

 

[LeeS69]

"Shot yourself in the foot twice" - what are you talking about. "This must mean every car on the road is technically speeding" - gibbering nonsense

And tell that to RCZ-R who mentioned citroens and 508s speedos

 

[Pedro]

Give it a rest your utterly boring!

 

[Neville]

My R over reads by 4kph and is an even margin at all speeds