function compute(form){

// get Index value for width
var wt = (form.width.options.selectedIndex)
var wt2 = (form.width2.options.selectedIndex)

// do crazy math to convert index no. to width in mm
var w = 145 + wt * 10
var w2 = 145 + wt2 * 10



// get Index for Aspect
var at = (form.aspect.options.selectedIndex)
var at2 = (form.aspect2.options.selectedIndex)

// do crazy math to get Aspect Ratio
var a = 30 + at * 5
var a2 = 30 + at2 * 5



// get Index for Wheel Diameter
var dt = (form.diameter.options.selectedIndex)
var dt2 = (form.diameter2.options.selectedIndex)

// do easy math to get Diameter
var d = dt + 12
var d2 = dt2 + 12



// all variables set, calculate total diameter in mm and inches
var mm = w * a * 0.02 + ( d *25.4 );
var mm2 = w2 * a2 * 0.02 + ( d2 *25.4 );
var i = mm/25.4;
var i2 = mm2/25.4;



// now the difference calculations
var mmc = mm2 - mm;
var inc = i2 - i;
var pr = (1 - (mm/mm2)) * 100;



// finally speedometer error
var sp3 = 30 + (pr * 0.3);
var sp30 = Math.round(sp3);
var sp6 = 60 + (pr * 0.6);
var sp60 = Math.round(sp6);
var sp9 = 90 + (pr * 0.9);
var sp90 = Math.round(sp9);

{

// send to form



form.mmtotal.value= mm;
form.intotal.value= i;
form.mmtotal2.value= mm2;
form.intotal2.value= i2;
form.mmchange.value= mmc;
form.inchange.value= inc;
form.perchange.value = pr;
//form.speedo3.value = sp30;
//form.speedo6.value = sp60;
//form.speedo9.value = sp90;

}

return;

}

function reset(form){

// ?

var w = 195
var a = 65
var d = 15
var w2 = 205
var a2 = 55
var d2 = 16

form.width.value = "";
form.aspect.value = "";
form.diameter.value = "";
form.width2.value = "";
form.aspect2.value = "";
form.diameter2.value = "";
}