simple coding problem

i have some code

static var speedometre = 1;

function OnMouseDown(){
    if (speedtest.speedometre == 1){
        speedtest.speedometre = 3;
        transform.Rotate(180, 0, 0);
        }
    else{
        speedtest.speedometre == 1;
        transform.Rotate(180, 0, 0);
        }
    }

But unity comes up with the error:

Expressions in statements must only be used for their side-effects.

What does this mean and how can i fix this error?

speedtest.speedometre == 1;

should either be

speedtest.speedometre = 1;

or

if (speedtest.speedometre == 1)