Hi JDD,
You are correct in that the wheel size is irrelevant, but I have a tyre spec fitted that is on the + side of the tyre standards sizing. Tyre makers are permitted +/- 4% for the OD of a tyre for a given size. It is the Rolling dia that you work the calculations from. That said, I have an electronic logbook fitted from my days in data logging devices and the speedo on my X5 over-reads by approx 5km/h everywhere including at 180. That is lazy work by BMW because they have offset the needle to read over everywhere to ensure they comply with design ADR's etc.
Recalibrating the computer calculation....I'm interested. I have other issues I need to sort with a custom re-cal like upshift points. Why the programmer felt the diesel engine needs to rev to 4500 for acceleration is beyond me. With a 6 speed box, 3800 would be tops with the current torque curve and the later X5’s with more gears are better programmed.
I just filled the tank up to the point where I can see the fuel and the calculation by odo and bowser and got 12.0L/100km. Trip computer said 11.3L/100km, so we are over-reading by 6%. Mind you I did not fill it up to the brim last time and I also was on level ground previously so the results are not exactly perfect.
Off to Sydney early tomorrow.
Cheers, Dan.