Relays get hot when switched too often in short time and are not as cheap as transistors that can switch at very high rate.
You don't need much current to drive the speedometer, signal strength FETs like 2N7777 could do for your 12V.
https://www.google.com/search?client=ubuntu&channel=fs&q=ss182+speedometer&ie=utf-8&oe=utf-8