[Daviey]

In the UK it certainly isn't by my interpretation. The Road Vehicles (Construction and Use) Regulations 1986, Regulation 35 & 36 is relevant.

Regulation 35 does allow compliance with alternative requirement, either Community Directive 75/443 or with ECE Regulation 39. These I am not familiar with, but the Regulation 36 regarding maintenance states:

"Every instrument for indicating speed fitted to a motor vehicle shall be kept free from any obstruction which might prevent its being easily read and shall at all material times be maintained in good working order."

Therefore, the original one would need to be removed to even begin to be legal.