To reset it to zero would be setting it to what the condition of the car would suggest it might have done.
That is exactly we are all firmly against when someone such as an unscrupulous dealer clocks a car to the mileage it looks like it might have done so as to increase the value.
How is setting a odometer to zero when the car has been fully restored and documented as such the same as someone
who clocks a car to hide the fact its done a high mileage to sell it on for profit?
Resetting mileage after a FULL engine rebuild using new parts and declaring the fact is legal, clocking is not legal.
You are absolutely right to say that the not declaring the mileage change is the illegal bit, its immoral too of course. I was only talking about how relevant the new mileage is to the car, the mileage in both cases reflects not how many miles have been covered but how many it looks like have been covered, that's the similarity.
Are you saying that mileage is there to indicate engine condition?
It isn't, putting a complete brand new engine in a car doesn't mean that its a zero mileage car, the odometer records miles travelled by the car not the engine, the engine is part of the car that can be replaced if necessary.
If I'm buying a car I would be perfectly happy to hear that its done 150k for example but had a full restoration at 120k. I would think it was a shame if the mileage had been reset. Of course as you said it would only be illegal or immoral if a seller now or later wasn't open about the mileage history and mislead a buyer.
It could easily happen that in a few years the car could be sold with say 60k on the zeroed clock and a buyer was allowed to think it was a genuine low mileage car from new, that's not OK is it, although at least nowadays the online MOT system makes it easy to track some of the history.