Speedo reads different to cruise control

[Mikrosoft]

Hi I have a 06 C5Mk2 1.6HDI and I have noticed that the speedo reads about 5mph faster than the what the cruise control is set at, anyone have any clue why

[oneday]

most of the speedo readings are out on the mk1 and mk2

[Mikrosoft]

Ahh I see lol so which one so I trust the speedo or the cruise?

Might have to get someone to follow me to know how far it's out.

[oneday]

i use the satnav to find out how far the speedo is out at 30 40 50 60 70mph its normaly a couple of mph on the high side so means i dont get any speeding tickets.

[Peter.N.]

My Mk1 C5 is spot on with the speedo reading according to the satnav, the first car I have had that is, most others have been around 10% fast so I suspect yours is.

Peter

[qprdude]

My speedo reads 71 mph (digital figure). Cruise reads 70 mph.

[Mandrake]

Ahh I see lol so which one so I trust the speedo or the cruise?

Might have to get someone to follow me to know how far it's out.

What makes you think the speedo in the car of the person following you would necessarily be any more accurate ?

Car speedo's are required to be +10% / -0%. In other words they're not allowed to read too slow but are allowed to read up to 10% too fast, so that any error in the reading will cause you to drive slower than the speed limit not faster than it.

The best way to to find out how far yours is out is to compare it to a GPS system - to do this you need to be in a clear area without tall buildings or trees and drive at a steady speed in a straight line for at least 10 seconds as GPS systems are slow to respond to speed changes. Although slow to respond to changes in speed, GPS systems are very accurate in absolute speed terms.

The speedo on my Xantia reads 70mph when actually doing 64mph as reported by GPS, this is pretty typical of many cars. When in 60/70mph zones I know I can add another 5mph on the speedo reading without exceeding the limit...(and even then everyone still overtakes me if I stick to that... )

[Mikrosoft]

Thanks guys I will set my satnav up at the weekend and give it a blast up the motorway and see where I am with it

[Old-Guy]

Mandrake's not quite right. The Construction and Use regulations specify that speedometers must read, for cars first reg before 1/2/2002 (IIRC) +10% -0%, after that date it's +5%, -0%.

To check with a Sat Nav, it's essential to maintain a constant speed for a couple of minutes on an open road with no buildings, mountains, trees or trucks etc partially obscuring the sky. To maintain user confidence, vehicle Sat Nav software does cunning averaging and predicting to calculate position and speed when satellites are hidden from view - most need between 4 and 7 satellites 'in view' to produce accurate results. Anyone who has used a Sat Nav extensively will have noticed the map occasionally showing a position behind or ahead of where you can see you are - most noticeable at some complex motorway junctions when the software under-estimates the sudden change in speed.

[DickieG]

My speedo reads 71 mph (digital figure). Cruise reads 70 mph.

That can be altered using a Lexia, when looking at the cruise menu it allows a set MPH differential between the cruise and actual speed, lower that figure to zero and they will both read the same, you'll then notice that the cruise is a little more responsive in it's desire to match the set speed so you'll then rarely see the speedo reading below the set cruise speed.

[waynedance]

My mk1 is the most accurate speedo I have had on any car.

[qprdude]

I've never bothered about it before. This is the first car I've owned with cruise and a digital speed reading. Couldn't say I would be able to get a reading to within 1 mph from a conventional dial speedo.
It's the old eyes you know.

[micky707]

I have posted this a few days ago, but i picked up a 56 plate C5 just over a week ago and had my suspicions about my speed. By running with the gps it turned out speedo was Under reading by 8mph! Just intuition that made me drive a little slower. Looking at the speedo and rev counter with the engine off the speedo needle was pointing lower than the rev counter needle, looks like it had slipped round a tad, I pulled off the console and moved needle back to 0 and bingo, now slightly over reading which is where i like it. So far well happy with car, just a gaitor split on suspension, should'nt be too bad to sort.