function f_calcSpeed() {


	var v; //hastighet i km/h
	var u; //rullsträcka för däck
	var a; //utväxlingstal
	var n; //varvtal
	var o; //omräkningstal km/h

	o = 0.06

	var prim, prim1, prim2;
	var sek, sek1, sek2;

	var oNumV,oNumR;

	n = document.forms[0].varvtal.value;

	if (document.forms[0].rullstracka.selectedIndex != 0) {
		u = document.forms[0].rullstracka.options[document.forms[0].rullstracka.selectedIndex].value;
	}
	else {
		u = document.forms[0].rullstracka_fri.value;
	}


	if(document.forms[0].ratio_fri.value.length > 0) {
		v = u * n * o / document.forms[0].ratio_fri.value
		oNumR = new Number(document.forms[0].ratio_fri.value)
	}
	else {

		if(document.forms[0].primar.selectedIndex > 0) {

			prim1 = document.forms[0].primar.options[document.forms[0].primar.selectedIndex].value.split("/")[0];
			prim2 = document.forms[0].primar.options[document.forms[0].primar.selectedIndex].value.split("/")[1];
		}
		else {
			prim1 = document.forms[0].primar_fri1.value;
			prim2 = document.forms[0].primar_fri2.value;

		}

		sek1 = document.forms[0].sekundar_1.value;
		sek2 = document.forms[0].sekundar_2.value;

		prim = prim1 / prim2;
		sek = sek1 / sek2;

		a = prim * sek
		v = u * n * o * a;
		oNumR = new Number(1 / a)
	}

	if (isNaN(v)) {
		alert('Please enter realistic values in all places!')
	}
	else {
		oNumV = new Number(v)
		document.forms[0].hastighet.value = oNumV.toFixed(2);
		document.forms[0].ratio.value = oNumR.toFixed(2);
	}

	return true;

}