Speedo Calibration

[Guest mcramsay]

My stack speedo only has an auto cal function, so to calibrate you have to put it into calibration mode, drive two miles and then press the button again to save the cal ( the lcd screen shows the amount of pulses it had counted during this process, so I can't drive the car before iva, so legally can't calibrate the speedo, I was thinking I know the wheel circumference, and thus can calculate the revouloutions required to travel 2 miles. If I multiply this by the no of magnets on the prop I should be able to work out the pulses the speedo should see in 2 miles. If I put the car on axle stands and put the speedo on to calibrate and then stopped the calibration when the Lcd screen shows the same number as my calibration do you think that will work?

[knights_templar]

If your magnets are on the prop you will have to factor in diff ratio

[Guest mcramsay]

Magnets are on one of the driveshafts

[cb750]

I think you will find that the actual rolling circumference when the load is on the wheel will be different to the theoretical one. Other than that your idea should get you close and you could always adjust on the way to IVA if you are driving there.

[maccavvy]

Can't see why it won't work..

Loaded or unloaded a circumference is the same.

[cb750]

The tyre radius changes when the weight of the car is on it therefore the rolling circumference will change

[maccavvy]

I beg to different..a rotation is still a rotation.surely.

With weight on it may squash but that stays the same as it rotates

A lower profile will rotate more over a given distance .

I'll happily be proven wrong though

[tractor]

I'm having speedo problems aswell think the sensor has had it

[cb750]

The rolling diameter (2 x r) will be smaller (not by much) with the load on the wheel so the effective circumference will be smaller (2 x pi x r). More rotations of the wheel would be needed to cover a mile so more pulses generated and would be needed to be accounted for in the calibration.

 

When I got the speedo calibrated on the stag and a couple of other cars I had to mark both the tire where it touched the ground and the road then push the car for 10 revolutions of the wheel and Mark the road again. The distance between the 2 marks was measured and used by the manufacturer in a formula to get the accurate number of pulses per mile.

[maccavvy]

I got my speedo from etc and their calibration involved counting rotations knowing the circumference. I wonder what difference would be if you pumped the tyre really up.any ?

[fry61]

The speedo reading will be more on a loaded tyre than an un-loaded tyre ( given a smaller circumference ) --- therefore & quid pro-quo -- fat blokes can see they are going faster than thin blokes.

[Guest mower man]

I set the ETB elect speedo on my hood in the garage with the car on 4 axle stands ,when checked out with sat nav it was bang on ----but was it set for airspeed ? mick