Speedo miles out

[mikearwas]

Evening all,

Having hooked up my satnav to my 130i I was able to check the accuracy of the speedo. I was amazed to see that at 90 leptons the speedo was 7 miles per hour out!! What could the issue be here? At higher speeds (of course only achieved on a track) it is far worse. Why is it so far out and has anyone else experienced this problem? At this rate the limiter would be cut in at about 135mph which is just not cool.

Any thoughts?

[vescaegg]

Size of wheels been changed from standard?

[VR6 Turbo]

is it over reading? this is pretty standard most cars are a round 5mph over. I could be wrong but it think its a percentage so the higher you go the more it over reads. not sure if your limiter takes that figure though. my Z4 used to go into the '160s'

VR

[barky]

speedos aren't allowed to under-read & I presume with wear could well read lower over time as odometer climbs beyond 100K+ ... so they over-read as standard to keep em legal .... think current cars read closer to the real speed than once was the case

[gamefreaks]

In most cars I've had the speedo woudl appear to have been deliberately set up to overread.

Try this. Drive at a constant speedo 70mph. Reset the trip computer. What does the average speed reading say? On the cars I have had, the average speed reading is pretty accurate even if the speedo is 5mph over so so.

[Willy Nilly]

a calibrated speedo will be spot on. It is quite possible that the GPS is inaccurate.

[2 sMoKiN bArReLs]

Also...GPS are on average 3% out due to gradient. (imagine if you were doing 70 mph vertically, gps would score 0mph)

[kambites]

That's about normal. Perhaps a bit further off than average, but not much.

[The_Burg]

As long as the speedo reads over it's ok.
Only accurate car i had was a V70 T5 which was bang on. (At least up to aroun 145 leptons).

[MartinQ]

That's to be expected, assuming it's overreading - i.e. showing 75 when doing 70. If you're that worried go with a friend on the motorway and get them to time you between the 100m markers. s=d/t and all that physics stuff...

[Tycho]

2 sMoKiN bArReLs said:

Also...GPS are on average 3% out due to gradient. (imagine if you were doing 70 mph vertically, gps would score 0mph)

Really, most GPS systems now will give you a 3d lock which gives altitude as well.

[2 sMoKiN bArReLs]

Tycho said:

2 sMoKiN bArReLs said:

Also...GPS are on average 3% out due to gradient. (imagine if you were doing 70 mph vertically, gps would score 0mph)

Really, most GPS systems now will give you a 3d lock which gives altitude as well.

Coolio. How's your shower curtain?

[Tycho]

2 sMoKiN bArReLs said:

Tycho said:

2 sMoKiN bArReLs said:

Also...GPS are on average 3% out due to gradient. (imagine if you were doing 70 mph vertically, gps would score 0mph)

Really, most GPS systems now will give you a 3d lock which gives altitude as well.

Coolio. How's your shower curtain?

Eh? With a 3d fix, you could be going vertical and you'd get an accurate speed reading.

[kambites]

Tycho said:

Really, most GPS systems now will give you a 3d lock which gives altitude as well.

My handheld GPS certainly does, I have no idea about my sat-nav.

[PoleDriver]

All of the post 1995 cars I've owned have read 4-6% high, except for one... The speedo on the Cerbera agrees with the sat-nav (within less than a needle's width) from 20MPH to 'very fast'!

I wish I could believe that the other gauges on it were as accurate!

[Nedzilla]

I have found in several cars that at motorway speeds the car speedometer usually reads around 7mph more than the gps reading.I always assumed the gps was accurate and the car manufacturers did this deliberately to avoid any hassle with people blaming their cars speedo for speeding offences.

[PoleDriver]

Nedzilla said:

I have found in several cars that at motorway speeds the car speedometer usually reads around 7mph more than the gps reading.I always assumed the gps was accurate and the car manufacturers did this deliberately to avoid any hassle with people blaming their cars speedo for speeding offences.

Correct!

[kambites]

It's also interesting to note that if I take the speed reading straight from the ECU, it's within 1mph of my GPS. It's only the needle on the dashboard that misreads... shows that it's intentional I guess.

Annoyingly, the odometer uses the same reading as the speedometer, not the one from the ECU.

[Willy Nilly]

I calibrated the speedos on some combine harvesters many years ago. The GPS yield mapping system gave forward speed as well, they never agreed.

The GPS on my phone says I am sat 84 meters above sea level and I've dropped 3 meters while typing this. I need to drop about another 40 meters at least before it is anywhere near right.

[2 sMoKiN bArReLs]

Tycho said:

Coolio. How's your shower curtain?

The dude from Tycho was sent to nick for fraud..he also spent £30,000 on a shower curtain...