What are my wheels worth?

[Danglesberries]

ALL speedo's over-read, it's a law requirement in the UK.

 

[Nictrix]

Why is it "wrong " ?
Bmw no longer fit 255 30 19 tyres so perhaps they realised they were pretty much alone amongst the other German manufacturers fitting 255 30 19 to a 9j rim .
You clearly haven't felt any noticeable impairment to handling ( why would you ) & another benefit along with comfort & wheel protection is the speedo will now be more accurate .

I thought the circumference of the front and rear axles should be (pretty much) the same

More important for 4 wheel drive systems

 

[Ed.Straker]

Why is it "wrong " ?
Bmw no longer fit 255 30 19 tyres so perhaps they realised they were pretty much alone amongst the other German manufacturers fitting 255 30 19 to a 9j rim .
You clearly haven't felt any noticeable impairment to handling ( why would you ) & another benefit along with comfort & wheel protection is the speedo will now be more accurate .

I thought the circumference of the front and rear axles should be (pretty much) the same

The primary issue is the stability control system..bmw call theirs DSC?

The system needs to know to a very high degree of accuracy what the absolute and relative speeds are of each of the 4 wheels.

Other sensors are required but fir this discussion they are not relevant.

Speed is calculated based on the abs sensor rings on each wheel being counted.

Then the ES computer then uses these rotations multiplied by the rolling diameter of each wheel to calculate absolute speed and relative speed of each wheel.

ES computers assumes that the wheel diameters are constants..it looks like the E89 assumes each corner has an identical rolling radius.

However there is no reason why different sizes front and rear rolling radius cannot be used providing the ES computer knows that.

For the E89 the DCS ‘how it works’ stated that each wheel should be with 1.5%

So by exceeding these parameters it’s probable that the ES system will not work as intended at certain events.

 

[Scooba_Steve]

ALL speedo's over-read, it's a law requirement in the UK.

It's not a requirement to over-read; rather not under-read and an allowance above:

The Motor Vehicles (Approval) Regulations 1996
SCHEDULE 2
19 Speedometers
1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at all speeds mph. up the maximum design speed of the vehicle.

2. For all true speeds up to the maximum design speed of the vehicle, the true speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the maximum design speed if lower), the difference between the indicated speed and the true speed shall not exceed—

V / 10 + 6.25mph

where

V=the true speed of the vehicle in mph.

 

[Danglesberries]

Not a single speedo reading in a vehicle gives an exact speed. They are deliberately set to over read. Lorries are governed by a maximum of 56mph. Try following one on the motorway and your speedo will read between 59 and 60 mph. The true speed you are doing is 56mph as the lorry s driving (generally) at the very maximum permitted.