I have a D4 with OEM 20" wheels and 255/50R20 tyres, and bought a cheap set of OEM RRS wheels which are also 20", but are slightly wider rims, and are fitted with 275/40R20 tyres. Not surprisingly, when these are fitted to the car the speedometer over-reads by 10 km/h at 100 km/h, compared with the 4 km/h optimistic reading with the std tyres. In both cases I compared the car speedo to a GPS.

I have an IIDTool, and found a few settings in "Car Config -> Instrument Pack" that seemed relevant:
"Tire size (rolling circumference)"
"KPH off-set"
"Speed scaling factor"

As the first option seemed like a basic value that would fix both speedo and odometer calibration, I changed that to 2150 and wrote the config back. I haven't been able to test the odometer accuracy, but a quick run shows that the speedo inaccuracy (ie +10%) was unchanged.

Some web and forum searching has not turned up a definitive answer on the config settings that need to be changed to fix the odometer, speedo and fuel economy calibration for different tyre sizes. Any advise please?

Also a general question. Is there an idiots how-to's guide to using an IIDTool or similar to perform common tasks, like this? There is good info on using the tool itself, but a web & forum search has found bits and pieces but not a comprehensive how-to guide.

Thanks
David