Speedo reads 10mph faster

[sheep911uk]

My Speedo reads 10mph faster is there any way i can adjust it to read correctly?i measured it against the speed indicated on my sat nav!
Thanks

 

[Peter Empson]

Yes, I think there is on the oval dash (don't know about the earlier ones, but cant imagine it'd be a problem), its very crude but its just a case of moving the pointer on the spline. I think there might be a tool to do this from memory as otherwise its easy to do damage.
10mph out sounds not too far from normal at max speed but is a real concern if its lower down, have you fitted different diameter tyres perhaps?
Mine actually under reads by about 3-4mph at 40 which is rather irritating, but I'm used to it now...
Best regards,
Peter.

 

[chrisdenton]

How accurate is the Sat Nav? Mine does seem to vary.

 

[Lythgoe]

my sat nav always shows different to my speedo...whatever car im using. I have seen somewhere that this is a fact with most sat navs.

 

[Fen]

The sat nav speed will be close to very accurate whereas the speedo rarely will. Sat nav calculates speed based on the movement it detects from GPS positioning, the speedo counts the number of revolutions of the road wheel and shows a corresponding speed based on gearing that assumes a certain distance covered by each revolution. The road wheel will have a diameter and hence rolling circumference of x with a brand new properly inflated tyre. Once that tyre is almost worn out the tread depth will have reduced from 8mm to 2mm or less, making a difference of some significance to the rolling circumference that is not accounted for with a change in the speedo gearing. Also manufacturers err on the side of overreading to ensure people don't accidentally speed. It also makes their cars look faster.

 

[Hilux]

Manufacturers are allowed 7% speedo error (I believe) so one car doing a hundred will be showing a hundred and another doing the same speed will be showing 107 (in theory)

I wouldnt worry about it as long as you are aware and it reads high.

If it sits at 10mph at Omph then it may need to be moved on the spline.

 

[sawood12]

I'm surprised Sat Nav is considered to be so accurate since it only has a positional accuracy of 3 metres or so. To calculate speed it will sample your position several times a second and given there is a possible error of upto 3 metres for every positional calculation this would translate into a speed calculation error. However from a magazine article I read about a VMax session and from the results of Porsche speedo's are much more accurate than other manufacturers. At one V Max event a Ferrari 360 Modena had an indicated speed of 180mph but the radar tracked him at a mere 160mph. Even at 180mph the Porsche speedo's were within 10mph of the radar tracked speed.

 

[Fen]

I think GPS is a lot more accurate than within 3 metres even if that is the tolerance of the system used. I imagine it's more accurate placing a moving position than a stationary one also as it has not only the distance from the receiver to several satellites to triangulate with but also the rate of change of position to use also. When I run a TomTom and a Road Angel on the same dash they agree on speed to within much less than one mph. In other words when accelerating gently they read the same almost all the time (variance could well be down to different or out of sync sample rates) and at constant speed they always agree. GPS is the basis for the timing gear used these days by magazines like EVO etc. and they wouldn't have binned the fifth wheel method if it weren't sufficiently accurate using GPS.

 

[chrisdenton]

ORIGINAL: Fen

The road wheel will have a diameter and hence rolling circumference of x with a brand new properly inflated tyre. Once that tyre is almost worn out the tread depth will have reduced from 8mm to 2mm or less, making a difference of some significance to the rolling circumference that is not accounted for with a change in the speedo gearing

So given that sizes make such a difference, what wheel/tyre size combination will I need to convince the missus I'm doing seventy when I'm actually doing ninety. []

 

[tref]

I'll let you have my speedo! Early dash car reads 18-22mph when stationary... ah, actually, thats the wrong way round for you isn't it?

Tref.

 

[mik_ok]

ORIGINAL: chrisdenton

So given that sizes make such a difference, what wheel/tyre size combination will I need to convince the missus I'm doing seventy when I'm actually doing ninety. []

255/40/25. Get some spinnahs. []

 

[jamescharnley]

my subaru's speedo consistantly reads 10% over compared to my gps