Revisting this, it seems like such a simple feature to add for advanced users who have calibrated their odometers. let's rejuvenate this thread and see how many users would benefit from this feature. I'm one!

If you are using roadside mile markers, do so over a long distance.

I spoke to the government department responsible for them and a single mile marker here or there isn't that accurate due to placement limitations, for example an on-ramp to the highway.

I have larger diameter tires on the car and know the odometer error. I compare the trip odometer to a garmin log.