Other S80 Problem: Speedo not reading correctly

[Simon Jones]

What size tyres have you got fitted to the car? If they're not the same rolling circumference as it was originally supplies with, it could contribute to the discrepancy you're seeing. I fitted some winter tyres / wheels to previous XC70 and it made a noticable difference to the speedo and odometer readings.

[Darlo 1]

My s60 d5, straight flat road my speedo reads within 2mph of my sat nav, my v40 also read within 2mph of the sat nav.

[sparkeysteve]

Originally Posted by Simon Jones

What size tyres have you got fitted to the car? If they're not the same rolling circumference as it was originally supplies with, it could contribute to the discrepancy you're seeing. I fitted some winter tyres / wheels to previous XC70 and it made a noticable difference to the speedo and odometer readings.

It has standard steel wheels. Then changed to Volvo alloy wheels from an S80 breaker. No difference in speedo reading.

Vidal does come up with wheel size of 1990mm can this be the reason? And can it be changed to the correct size?

[Simon Jones]

It's the size of the tyres that determines the rolling circumference. What size have you got?

[sparkeysteve]

Originally Posted by Simon Jones

It's the size of the tyres that determines the rolling circumference. What size have you got?

Yes very good question. I'll get back to you on that one asap

[SwissXC90]

Originally Posted by sparkeysteve

It has standard steel wheels. Then changed to Volvo alloy wheels from an S80 breaker. No difference in speedo reading.

Vidal does come up with wheel size of 1990mm can this be the reason? And can it be changed to the correct size?

Let's do some math:

Firstly, what wheels and tyres are right for your car?
A bit of sleuthing on the internet shows that most S80s have the folloiwng combinations permissable:
225/55R16
https://tiresize.com/tiresizes/225-55R16.htm
Circumference: 80.8 inches = 2052.3 mm

225/50R17
https://tiresize.com/tiresizes/225-50R17.htm
Circumference: 81.2 inches = 2062.5 mm




Most countries allow 0 kmph to around +4 kmph permitted variance.
This means the car may show 104 kmph when it is actually traveling 100 kmph
This is to ensure the driver does not blame speeding on instrumentation errors.

We can use +4% as a rough typical guide to maximum permissible speedo error, although some legislations state %, and some give the variance in kmph.


So lets calculate the error resultant in your S80
Lets look at 225/55R16 - it has a circumference of 2052 mm, which is 3% higher than 1990
This means your speedo reads 3% higher than the calibration setting of 1990 mm, within the permitted limits.

What happens when the tyre wears?

A type with circumference of 2052 mm has a diameter of 653.2 mm (basic math)
The speedo calibration of 1990 mm assumes a minimum rolling tyre diameter of 633.4 mm

So you can see that as the tyre wears from 653.2 mm diameter to 633.4 mm diameter, meaning a tread loss of 19.8mm over the diameter or 9.9mm over the radius (tread occurs on the entire circumference, so tread wear is a change in radius).

New tyres have a tread depth of typically 6 to 8 mm
So you see that when a new tyre is completely worn down from 8 mm to 0 mm tread depth, the diameter changes from 653.2 - (2 x 8) = 637.2, which is 2001 mm circumference, and still 0.5% higher than the calibration value, and thus still permissible


Conclusion:
With a calibration value of 1990 mm stored, and with 225/55R16 tyres fitted on 16 inch rims, the speedo will read 3% higher on new tyres, reducing down to 0.5% higher as the tyre tread depth wears.

The above math shows that your car is setup for 225/55R16 or other wheel / tyre combinations with new tyre circumferences of 2052 to 2069 mm (+4% over 1990)

I trust you enjoyed the high-school math examples used above

[SwissXC90]

Update:
I just checked the legislation in my country (gotta love the Internet)
Here the allowed deviation is -0% to +10% betweeen 40kmph and 120kmph, plus a fixed 4 kmph

In the UK, the allowed deviation is -0% to +10% (I don't have any greater data than that)

So allowed deviation is:
Actual speed: 40 kmph
Permitted Speedo Display: 40 to 44.4 kmph in CH, 40 to 44 kmph in the UK

Actual speed: 120 kmph
Permitted Speedo Display: 120 to 136 kmph in CH, 120 to 132 kmph in the UK

Deviations greater than the above are not permitted, meaning a vehicle check failure.