Just to confirm:

• I guess the attached DataContext exposes a "Mileage" property
• Is the binding working if you remove the StringFormat?

yes, binding is correct and also some other formats, like 0:N6 work well

I've checked the specification and it turns out that D specifier only works with integer values. And the issue is all the number properties in the Data structures are float values, so the string format fails even if there are no decimals.

As a workaround you can use the following custom format:
Text="{Binding Path=Mileage, StringFormat={}{0:000000} KM}"

Could you try that?