I was inordinately pleased, like a small child with a new toy, to discover this page on Teh Intarwebs the other day. It gives childishly simple instructions for turning one's VW Climatronic system into a diagnostic interface to the ECU and other sub-systems, and then on interpreting what 2 little numeric LCDs are telling you.
What I found most interesting about this wasn't a dynamic update on the AirCon's refrigerant pressure (2.8 bar rising to nearly 6 as the system got up to speed) but that the diagnostics provides a readout of values you can see on the main dials - RPM, Speed, water temperature etc. The speed one was *most* interesting.
As a matter of law, all speedos are required to *never* under-read speed. Because nothing's ever absolutely accurate, in practice this means that when your car says you're doing 70, you're probably doing something between 66-69 MPH (there's a 10% variance allowed provided the speedo is over-reading). This is fine, we can all accept this from a safety perspective. Those of us nerds with external speed measuring devices - or GPS / SatNavs as they're more commonly known - have observed this effect for years. My car's speed is adjusted such that 74MPH indicated (and set on the Cruise Control if I wish) is 70MPH true, according to the (accurate to ~1MPH in steady-state conditions) Sat Nav I've got on the dash.
But here's where it gets interesting: The car's diagnostic screen, value 19-1 gives current speed in MPH. And from observation this *exactly* matches SatNav. So with cruise at 74, speedo at 74 indicated, this diagnostic shows 70. In other words, the sensors in my car (it's all electronic these days) go to the trouble of picking up an accurate external speed (by wheel rotation speed no doubt), which is probably recorded and used against me internally, but then goes to the trouble of adding on 10% or so for display on the speedo dial.
Or put it more accurately: My car is actively lying to me about how fast I'm going.
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