Intermittant Rev Counter, Econometer,

[blimp]

My rev counter and econometer seem to be having issues - Both seem to play up after the car has bee driven for an hour or so and either

- both stop working at the same time, or

- econometer stops at its off position (past the 50 mark) while the rev conter works, or

- econometer stops and the rev counter flicks wildly for a minute or so then stops ,or

- economteer sticks around the 20 mark while rev counter works.

Is this going to be my SI board batteries? The SI has the yellow led lit, and doesn't do anything strange. If it is the SI batteries on their way out would it produce symptoms all the time? These issues only seem to appear after the car has been driven for a while and is warmed through.

Also while while using my sat-nag today the speedo appears to be under reading by about 4 mph at nigh on any speed (26 on satnav, 30 on the speedo, 56 on satnav 60 on speedo). Is this normal?

[Brianmoooore]

Under reading speedo is not only normal, it's law! The speedo must not under read under any circumstances, as you could inadvertently find yourself guilty of a speeding offence if it did.
The purpose of a speedo is to enable you to drive within the law, not to tell you how fast you are going.
I can't say for certain that your SI board batteries are failing, but it's the first thing to check.
Pull the board out, stick it in the freezer for a quarter of an hour, then measure the voltage across each battery.

[blimp]

I think I confused myself - if my speedo is saying I'm doing 30 when sat nav says 26 then isn't that over reading? Which sounds better as I'd only be holding traffic up and not exceeding the limit.

I'll have to have a bash at dismantling the dash next weekend to check the si board and batteries

[Brianmoooore]

If you think you are doing 30 when in a 30MPH limit, and you are actually doing 26, then you aren't going to get a ticket.
If you follow anyone with 9+ points on there licence through a 30 limit, and check their speed against a calibrated speedo or GPS, then it's very likely they will be going at around 26.
If you don't have 9+ points on your license, most people will 'push' the 30 limit a little, and travel at what they think is around 35 MPH, which in reality is little more than the limit.

[bpowell555]

You're ok up to the limit plus 10%, so 33 in a 30, which given the speedo's safety threshold means 35 should be ok. Accordingly 45 in a 40, 55 in an average speed 50, 75 on the M25 etc.

[blimp]

I only asked as the speedo on my last car (a 90 Peugeot 309) was spot on when I used the nav.

[Brianmoooore]

Then it's the Pug speedo that is wrong, if your tyres had a decent amount of tread on them.
With nearly bald tyres and on a wet surface with more slippage, it would have under read.
Having has experience of both Pug electrics, and BMW electrics, I can assure you that there is absolutely no comparison!
You don't find BMW positioning an open construction crank sensor plug just by the battery vent, or positioning the main engine loom connector right behind the gap beside the headlamp, where all the rain and muck comes through.