if you want dodgey, silicon a magnet onto the inside of your drivers side wheel and then hook up a bike Speedo. Works a treat as they are easily adjustable. Did it on two previous cars additional to the car Speedo. You can even get back lot ones.

I know one other member that had done it too. I did it originally on mini in the late 1990s to check the Speedo calibration. Left one as a permanent fixture in my old inre so I wouldn't get speeding fines as it was so accurate. Might do it again lol.