I've been playing around trying to drive my mechanical speedometer via a motor due to calibration being out since changing gearbox and differential ratio.
Now realised I've too many projects on and any which can be sorted 'professionally' particularly if no great cost is involved should be done this way. To that end I want to send my speedo' off and get it calibrated.
To do this I have to work out how far the car moves for one revolution of the wheel as well as counting the speedo' cable turns for a given number of wheel revolutions. I've done the second part but unfortunately I'm not in a position to move the car and measure the distance. I've worked out the theoretical value but this won't be accurate due to the reduction in tyre radius when the weight is on the wheel so I'm just wondering if anyone has done this measurment for 225/55 R16 tyres as had the figure available?
As always any help greatly appreciated.
This is a good website for tyre details. I used this to set up my speedo.
https://tiresize.com/tiresizes/225-55R16.htm
225/55R16
Diameter : 25.7"
Width : 8.9"
Wheel : 16" x 6-8"
Sidewall : 4.9"
Circum. : 80.8"
Revs/Mile : 784
Quote from: JING on Mar 03, 2024, 02:42 PMThis is a good website for tyre details. I used this to set up my speedo.
https://tiresize.com/tiresizes/225-55R16.htm
225/55R16
Diameter : 25.7"
Width : 8.9"
Wheel : 16" x 6-8"
Sidewall : 4.9"
Circum. : 80.8"
Revs/Mile : 784
Thanks
@JING however I think they are 'ideal' reflecting the unloaded size of the tyre as they are very similar to my own measurements taken from the wheel but when the weight is on the centreline to road surface is going to reduce and with it the rolling circumference.
Just to close the loop on this in case it helps somebody out in future, I've now done a physical test on the car so have an accurate figure.
The figure I've measured for circumference is 2012.5mm
This compares to the 'ideal' figure of 2054.6mm or a factor of 97.95%
This was taken over 4 revolutions of the wheel divided by 4 to reduce any measurement error.
As a double check I measured the 'ground to axle centreline' which as 320mm, working this back from the circumference measurement should give 320.299mm hence viable.
This was on a Gardner Douglas which has an all up weight of around 900KG (no driver present, I was pushing it!) and 23PSI in brand new (full size) 225/55 tyres.
Hopefully this may help anyone in similar situation either directly if they have same size tyres or possibly give a good comparison if the ideal:actual factor is used with different sizes.