Ian Vincent Posted April 8, 2019 Report Share Posted April 8, 2019 (edited) 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 April 8, 2019 by Ian Vincent Quote Link to post Share on other sites
Drewmotty Posted April 8, 2019 Report Share Posted April 8, 2019 Mine reads spot on as close as I can gauge using a timing light with a rev counter function. Quote Link to post Share on other sites
kiwican Posted April 8, 2019 Report Share Posted April 8, 2019 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 Quote Link to post Share on other sites
barkerwilliams Posted April 8, 2019 Report Share Posted April 8, 2019 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 Quote Link to post Share on other sites
ed_h Posted April 8, 2019 Report Share Posted April 8, 2019 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 Quote Link to post Share on other sites
jerrytr5 Posted April 8, 2019 Report Share Posted April 8, 2019 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 Quote Link to post Share on other sites
DRD Posted April 9, 2019 Report Share Posted April 9, 2019 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 Quote Link to post Share on other sites
Ian Vincent Posted April 9, 2019 Author Report Share Posted April 9, 2019 I'm having my engine set up on a rolling road towards the end of April so no doubt I shall find out the truth then, but I'm glad it's not just me. Rgds Ian Quote Link to post Share on other sites
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