Something else maybe worth mentioning:
After updating the tire size; while driving for that initial 20 minute drive the engine fan was on and would not shut off.
I pulled over shut the truck off, pulled the relay and then replaced it (not with a new one). Started the truck, drove off and the fan was no longer running (as it shouldn’t at operating temp) and then about a 1/4 mile later is when I got that code: ABOVE POST

 

Found this

https://www.rangerovers.net/threads/p0501-error-code.306609/

Helpful, but doesn’t really point to a definitive answer.
my radius setting is 406. Might just adjust to 388 like that thread suggests. My tire size is 255/70r18. 32” just like in that post

 

No Gap tool, here. 2011 with the HSE Luxury 20" wheels.

Moved from the OEM 40-profile to 50 profile tires, did the math, reminded meself the speedometer would be under actual speed by a skosh, and just drove it.

Not a huge difference, but NO codes of any sort related to any of that appeared. So there is at least 'some' margin of tolerance built-in to the system? Technology exists to allow it to determine the rolling parameters for itself, but I doubt that has been applied to L320's.

What happens if you set all parameters back to where they were before you changed the tires?

Annndd.. have ta' ask it...Surely all four corners have the SAME tires as well? And the CCF holds the same parameters for all?

This sounds like a "gross" or "out of bounds" conflict.
The sort a computer might act on if it confused radius and diameter, or not all tires match both on-road and in CCF, even.

Worth contacting Gap?

 

Great to find this thread, as i had the exact same issue — including the weird fan behavior. I had specified 394mm radius. I was able to make the CEL turn off by using 390mm instead. I drove the car with the CEL on for a week and the light never recovered.

 

Interesting. I got the same CEL when I changed the size in my Gap tool, so I just changed it back to default to make the light go away. I may try changing the size to just under the new size. I have 265/65/18.