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I have always recognised that the old fashioned type of speedo (as in the ones in our cars) will  only provide an approximation as to how fast one is travelling, by virtue of the way in which they work.  But..... for some reason I have always assumed that the rev counter (which as far as I can work out operates on the same principle) was gospel.

When calibrated against a sat nav my speedo over estimates by about 10% across its entire range.  At the weekend I decided to check the rev counter.  I measured the rolling circumference of one of the rear tyres and then calibrated the rev counter against the sat nav in each gear before dividing by the gear/final drive ratios.  It turns out that my rev counter is over estimating by about 10% as well.  At a steady 50mph in top (as indicated by the sat nav), the rev counter is showing pretty much dead on 2,500 rpm whereas the calculated figure should be 2287 rpm.  And as far as I can tell, the error seems to be consistent across the range.

So when I am trying to get the engine to idle smoothly at an indicated 750 rpm, I am actually trying to get it to idle at somewhere between 650 and 700, which probably explains why it is a bit lumpy. (it has a slightly hotter cam and a moderately lightened flywheel which probably doesn't help).

Has anyone else checked the accuracy of their rev counter - and how did it perform?

Rgds Ian

Edited by Ian Vincent
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I wonder if the sidewall flex where the rubber meets the road is enough to throw off the reading a little. Your math equation would assume a perfectly round tyre whereas the actual road to top of tyre height will a little different (less). Doubtful that makes 10% difference though.

 

Simon

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Electronic rev counters that pick up the pulse from ignition coil are available for less than £20. Five minutes with a  couple of lengths of wire and you can park the car  sit in the drivers seat and rev the engine and compare the two readings to determine any discrepancy. 

Alan

 

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The rev counter (tach) and speedo are essentially identical inside except that the tach lacks the odometer parts.  I believe they all read slightly high when new (at least the speedos--not sure about the tachs), but tend to degrrade over time.  Probably the magnets getting weak.  The instruments can be adjusted and calibrated at home with some setup effort.

Ed

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10% is about average I would guess.

When my efi converted engine was set up on the rollers for the rev limiter, that was about 10% out.

Jerry

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I've been checking this myself recently as when I had the engine setup on a rolling road I was told the tachometer was under reading, I.e. at 5500 it was reading 4300 which I was rather alarmed by. I've since used a multimeter to measure rpm independently but found it was under reading by 10% up to 3500rpm. So I'm not sure what's going on. Also I found the meter would fluctuate wildly as the engine sped up so was only good for a steady speed.

Daz

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