Speedo Error

[SplendiferousII]

I just happened to have the Sat Nav in the van on the way home form work and thought I'd check its speed reading against my van Speedo.

Way Out !

At 60 mph (Sat Nav which I think is going to be spot on accurate) the van is reading 68mph.

Cant be arsed to do anything to correct it but has anyone else checked their readings to find similr errors?

[maddog334]

yes mate at 65 speedo reading the sat nav is saying 56mph, I know my tyres are a wee bit smaller but should only give around 1% error, still I think its better for the speedo to over read.

Martin

[Pepperami]

Sat nav 68 speedo 70

[lloyd]

[quote="Pepperami"]Sat nav 68 speedo 70[/quote
about same here. Sat nav 70, 70 speed. Sat nav / speedo will very a couple miles/hour depending.

[campervanman]

Satnav will only give you a true spped reading on the flat and level. It will give a lower speed reading ging up and down hills.

[lambrettalee]

Satnav will only give you a true spped reading on the flat and level. It will give a lower speed reading ging up and down hills.

how do you work that out??

[Bilbo Blue]

Because the satalite don't work out your altitude at any given time. ie it only sees you going from point a to point b as the crow flies, it wont see you've gone over a mountain and in effect traveled extra distance.

My sat nave reads 60 an the van reads 63.
Exactly the same with me mates T4.

[asahartz]

But unless you're going down a 1 in 4, quite frankly it's insignificant.

Mine is way out too; 55 on the speedo is actually 50 on the satnav.

This happens in a lot of cars (my wife's Previa is similar, and most of my Minis), which is why some people seem to go way under the limit past speed cameras - they're doing what their speedo tells them is the limit.

[lloyd]

Satnav will only give you a true spped reading on the flat and level. It will give a lower speed reading ging up and down hills.

how do you work that out??

Distance covered is greater on hills then on level. Sat nav speed does not calculate the elevation change distance, only level earth surface. A 10% grade 1 mile on gps is 1.028 actual rolling distance, so 2.8% more distance or 2.8% actual faster speedo reading then Sat nav speed will show.

[windysurfer]

Mine show 65 on speedo and 62 on satnav.

Is there not a law about your speedo reading 4% fatser than actual speed?

Might be a US law but I'm sure we just followed suit

[lloyd]

Speedo can read fast, but not slow by law. Usually 2-4 mph faster then actual speed.

[Syncro G]

Mine show 65 on speedo and 62 on satnav.

Is there not a law about your speedo reading 4% fatser than actual speed?

Might be a US law but I'm sure we just followed suit

Between actual speed and +10% is what they must do (never checked though). In reality most speedos tend to be calabreated when new to overead +5 to +10% so 65 when doing 60 is perfectly accepterble. Having expereanced T3's from new they seem to like their speedos as optimistic as possable (that doesn't mean the odo is so far out though as thats set differently, and won't change through age (its direct gear drive so it can only break - no magnets and springs unlike the clock)). Change the tyre size and age the spring inside the clock (makes it read faster) and you can have any sort of error.

GPS errors on hills might not be massive but when you add them to the designed error in your speedo, plus the error through tyres or age and it adds up! I know some GPS devices give altitude but they are not very accurate when you test them and quite slow acting as they have to triangulate your position from lots of satalites right above you - not the best place to do it from. Have a look at the high end GPS devices and they'll have a seperate altomiter in them based on presure - needs regular calabration but its much more accurate (not sure if those GPS sets take that into account when calcing speeds though?)

[vwjon]

vw speedo's are 4% over compensated, ie 50mph and your speedo will read 52mph, the reason for this is if the speed limit is 50mph, you drive at 50mph, there is no way you will pick up a speeding ticket, or be breaking the law. vw are covering their arses and also yours! quite rightly so!

[Red Westie]

When you think about it, they need have some tollerance because of tyre tread wear effecting overall circumference.
You start off with a good 7-8mm and then wear the tyre down to 1.6mm and although it isn't much, it must effect the overall circumference by a few percent.
Martin

[mal0r]

My speedo bounces between 15 mph and 45 mph when I'm trying to do about 30, so I tend to only let it bounce as high as 30 or so when going past a camera