Fluctuating Speedometer

[ianc]

James - if Ian V had a 3.7 axle, your 58.6 at 2400 would be correct, but he has a 3.45 axle so he will get 3.7/3.45 x 58.6 = 62.8, or thereabouts.

So, his steady 60mph sounds correct.

Ian Cornish

[Ian Vincent]

I have checked my calculator against the one provided by Mr Minty and they don't align.  I suspect that it is because he is using a different rolling circumference to me.  I measured mine by rolling the car one complete revolution of the wheel and measuring the distance between the two marks on my drive - it came to 194.5cm, from there it is simply a bit of spreadsheet arithmetic.  I accept that when a tyre is rotating at speed there will be a tendency for it to increase in diameter but for my spreadsheet to align with Minty I needed to increase the rolling circumference to 202.5cm (i.e. an increase of 8cm) which seems rather a lot.  Everything else is spot on between Minty and my sheet so we are using the same method and there is no hidden non linear adjustment built into the Minty calculator.

My suspicion is that given that a mechanical rev counter is using the same crude technology as the speedo (i.e. a rotating magnet and a disc), my rev counter is over estimating the engine speed.  I do have a seperate electronic rev counter which uses a ferrite magnet around a plug lead so later today I'll give it a whirl and calibrate my rev counter.  I can then adjust my speed calculator accordingly

Edited November 26, 2022 by Ian Vincent

[Ian Vincent]

Well that didn't work!  I finally found time to check the engine speed on my electronic rev counter against the car instrument and I got a pretty consistent straight line between the two values when plotted with a gradient of about 1.32.  That suggests that at 2400 rpm on the car rev counter the actual rpm is only 1820rpm which is clearly wrong - the car rev counter is not the only thing that needs calibrating.

So I will have to wait until I get a suitable day for a drive out on a nice straight road and calibrate the speedo and rev counter against a decent sat nav.

Rgds Ian

[Charlie D]

Ian,

Do a search on eBay for :

DT2234C

It's a hand held rev counter ("DT2234C Handheld Digital Laser Rev Counter Meter Non-contact Optical Tachometer " the advert says.)

101 uses and only £12.

Charlie