Tyre size and circumference to adjust speed

[r1sm]

Hello. 

This is a public service announcement to help others.
My goal was to adjust the display speed to Waze GPS, so that I wouldn't always have to do the mental arithmetic to drive +3 km/h over correct speed (smooth traffic flow).

1. Tyre size
Changed it successfully to 235/50R19

1. Circumference
Calculated would have been 2253 mm, but that was far off. Read below what SPAyceTech shared on the subject. In short, Volvo does it's own thing here.
After much trial and error up and down with values I landed on 2120 mm. Seems to be a good fit.

The following information on the subject is from SPAyceTech.

The tire size and circumference are separate items in config, but Volvo uses them together, within the limited options offered in VIDA (only factory option tires and sizes are covered).
When you run a "tire size" software in VIDA, it sets both the tires size and circumference values.
Tire size is used for steering stops. This allows the fitting of larger wheels/tires and resulting adjustment of the maximum steering angle so the tires do not rub on the wheel liners, suspension parts etc. 

I do think the circumference config is what is used to correct for distance and speed calculations throughout the car, like the speed display on the dash, odometer distance etc.
There is no indication of the proper measurement of circumference. We don't know what Volvo is measuring and using as a value there. 
The values Volvo uses for a tire circumference don't usually match an online calculator. An online calculator is only a very generic rough guide of approximate dimensions of an actual tire.

Hope it helps someone else as well.
Cheers,
R

[IMAY2023]

Thank for the info. 
I guess in Europe it's different.  In the US I tested this with several different GPS software and all three of them indicated the same speed what Car's odometer shows. 
waze, google maps, Apple, IGO on Android.

[hen123drik]

i'm also trying to match the gps speed to my odo speed. For some reason, my car had a 21inch as default in the computer wehreas the tires under it are 255/40R19. The speed was off by approx.5km/h. 
I've adjusted the tire size (255/40R19) and matching circumference (2156) but now it is even more off. 

Using your input, i've calculated that the difference between your supposed circumference and the best fitting circumference is approx 6%. 
I'm also going to decrease the tire size in the car's computer with 6% (from 2156 to 2028) to see if that makes an improvement.

[x119]

You guys know that tyres change circumference as they wear right...?

I certainly wouldn't want it to read exact! A mph or 2 offset is good.

[IMAY2023]

Well as long as it does not show slower than the actual speed, it ok for me.   New speed cameras are being installed in all over CA cities that has 0 to 5 mph tolerance above posted speed depending on the zone.

[hen123drik]

The 2mph offset (My mind is wired to SI units so i'll stick to that) might be okay depending on the speed; if i have 5km/h offset at 20km/h, i find that a lot. If it is 5km/h offset at 150km/h, i can live with that.
anyhow, back to the topic; i want my speedo to be a speedometer, not a guess-o-meter. so i'd like it to be within 1-2km/h at 100km/h (1% deviation).

My theory didn't work, it was off by about 12%! so i thought i maybe calculated it backwards and my tire circumference should be 6% larger in the cars's computer. Again, no luck. I've now got it set on 2145 (theoretically they should be 2156; tires are brand new). Still about 5% off... I might try a couple of steps lower than that (2140, 2135, 2130) to see if that gives an improvement.

To get back to the initial post; it seems that volvo indeed does it's own thing and that certain tire size and circumferences do not compute correctly. It's a bit of an iterative excercise get the speedo to show actual speed. I'm even thinking that changing the 'vehicle type' to police car might unlock better speedometer calibration, but don't want to try that one out just yet.