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Originally Posted by fairBA
correct me if I'm wrong but when certain models depoly air bags - the speedo and tacho stay frozen displaying the speed and revs at the time of the crash - A few cop/crash shows feauturing bseries have done this...
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I have been to many crashes as part of my work and I can not remember seeing a speedo or tacho "frozen" at the last reading on a car.
I assume this is because most speedos and tachos work electronically and at the loss of the electrical signal, the needle will return to zero as a result of the spring pressure. Try unplugging your instrument facia from the wiring loom with the engine running to demonstrate this. I have heard of this freezing of the reading happening on older vehicles with cable drive speedos and tacho but there are not many of these vehicles now.
I have however seen a motorcycle that had a mechanical drive tacho that froze at the last reading on impact when it t boned a car. The accident investigator said that they will check what gear the bike was in and calculate the speed according to the reading on the tacho and the gear ratio. Considering the bike had a heavy aluminium box section frame on a high powered sport bike, which was snapped in two at the head tube, he was going a bit faster than the 60 kmh limit.
Next time I am at a serious crash and accident investigation are there, I will ask to clear this up.