[devote]

Я вижу только два места куда вы можете вставить свой код, указал комментариями:

(function() {
    var oSheme, oCalculator, oDiscCalculator, oSpeedometer = [];

    window.onload = function () {
        initLegend();
        initCalculator();
        initDiscCalculator();
    }
    function initLegend() {
        var elem = document.getElementById('shemeLegend');
        if (elem) {
            oSheme = {
                'nLegend': elem,
                'aLabels': []
            }
            initLegendLabel('L');
            initLegendLabel('H');
            initLegendLabel('D');
            initLegendLabel('DD');
        }
    }

    function initLegendLabel(sLabel) {
        var elem = document.getElementById('labelSheme' + sLabel);
        if (oSheme && elem) {
            elem.onmouseover = function () {
                legendMouseOver(sLabel);
            };
            elem.onmouseout = function () {
                legendMouseOut();
            };
            oSheme.aLabels.push(sLabel);
            if (elem = document.getElementById('sheme' + sLabel)) {
                doPreload(elem.src);
            }
        }
    }

    function legendMouseOver(sLabel) {
        legendMouseOut();
        addClass(oSheme.nLegend, 'label' + sLabel);
    }

    function legendMouseOut() {
        for (i in oSheme.aLabels) {
            removeClass(oSheme.nLegend, 'label' + oSheme.aLabels[i]);
        }
    }

    function initCalculator() {
        oCalculator = {
            'oldWidth': document.getElementById('oldWidth'),
            'oldDiameter': document.getElementById('oldDiameter'),
            'oldProfile': document.getElementById('oldProfile'),
            'newWidth': document.getElementById('newWidth'),
            'newDiameter': document.getElementById('newDiameter'),
            'newProfile': document.getElementById('newProfile'),
            'oldL': document.getElementById('oldL'),
            'newL': document.getElementById('newL'),
            'deltaL': document.getElementById('deltaL'),
            'oldH': document.getElementById('oldH'),
            'newH': document.getElementById('newH'),
            'deltaH': document.getElementById('deltaH'),
            'oldD': document.getElementById('oldD'),
            'newD': document.getElementById('newD'),
            'deltaD': document.getElementById('deltaD'),
            'oldDD': document.getElementById('oldDD'),
            'newDD': document.getElementById('newDD'),
            'deltaDD': document.getElementById('deltaDD')
        }
        var elem, i = 10;
        while (elem = document.getElementById('speed' + i)) {
            oSpeedometer[i] = elem;
            i += 10;
        }
        applyCalculateEvent(oCalculator.oldWidth);
        applyCalculateEvent(oCalculator.oldDiameter);
        applyCalculateEvent(oCalculator.oldProfile);
        applyCalculateEvent(oCalculator.newWidth);
        applyCalculateEvent(oCalculator.newDiameter);
        applyCalculateEvent(oCalculator.newProfile);
        doCalculate();
    }

    function applyCalculateEvent(nNode) {
        nNode.onchange = function () {
            doCalculate();
            // либо сюда вставляете свой код
        };
    }
    function doCalculate() {
        var iOldL = oCalculator.oldWidth.value;
        var iNewL = oCalculator.newWidth.value;
        var iOldD = Math.round(oCalculator.oldDiameter.value * 25.4);
        var iNewD = Math.round(oCalculator.newDiameter.value * 25.4);
        var iOldDD = Math.round(
            oCalculator.oldWidth.value *
            oCalculator.oldProfile.value * 0.02 +
            oCalculator.oldDiameter.value * 25.4
        );
        var iNewDD = Math.round(
            oCalculator.newWidth.value *
            oCalculator.newProfile.value * 0.02 +
            oCalculator.newDiameter.value * 25.4
        );
        var iOldH = Math.round((iOldDD - iOldD) / 2);
        var iNewH = Math.round((iNewDD - iNewD) / 2);
        var iSpeedCoeff = iNewDD / iOldDD;
        setTextValue(oCalculator.oldL, iOldL);
        setTextValue(oCalculator.newL, iNewL);
        setTextValue(oCalculator.deltaL, iNewL - iOldL);
        setTextValue(oCalculator.oldH, iOldH);
        setTextValue(oCalculator.newH, iNewH);
        setTextValue(oCalculator.deltaH, iNewH - iOldH);
        setTextValue(oCalculator.oldD, iOldD);
        setTextValue(oCalculator.newD, iNewD);
        setTextValue(oCalculator.deltaD, iNewD - iOldD);
        setTextValue(oCalculator.oldDD, iOldDD);
        setTextValue(oCalculator.newDD, iNewDD);
        setTextValue(oCalculator.deltaDD, iNewDD - iOldDD);
        for (i in oSpeedometer) {
            setTextValue(oSpeedometer[i], Math.round(i * iSpeedCoeff * 10) / 10);
        }
    }

    function initDiscCalculator() {
        if (
            document.getElementById('tireWidth') &&
            document.getElementById('tireDiameter') &&
            document.getElementById('tireProfile') &&
            document.getElementById('discDiameter') &&
            document.getElementById('discWidthMin') &&
            document.getElementById('discWidthMax')
        ) {
            oDiscCalculator = {
                'tireWidth': document.getElementById('tireWidth'),
                'tireDiameter': document.getElementById('tireDiameter'),
                'tireProfile': document.getElementById('tireProfile'),
                'discDiameter': document.getElementById('discDiameter'),
                'discWidthMin': document.getElementById('discWidthMin'),
                'discWidthMax': document.getElementById('discWidthMax')
            }
            applyCalculateDiscEvent(oDiscCalculator.tireWidth);
            applyCalculateDiscEvent(oDiscCalculator.tireDiameter);
            applyCalculateDiscEvent(oDiscCalculator.tireProfile);
            doCalculateDisc();
        }
    }

    function applyCalculateDiscEvent(nNode) {
        nNode.onchange = function () {
            doCalculateDisc();
            // либо сюда вставляете свой код
        };
    }

    function doCalculateDisc() {
        if (oDiscCalculator) {
            var iWidth = oDiscCalculator.tireWidth.value;
            var iProfile = oDiscCalculator.tireProfile.value;
            var iDiameter = oDiscCalculator.tireDiameter.value;
            var iWidthMin = (Math.round(((iWidth * ((iProfile < 50) ? 0.85 : 0.7)) * 0.03937) * 2)) / 2;
            var iWidthMax = (iWidthMin + 1.5);
            setTextValue(oDiscCalculator.discDiameter, iDiameter);
            setTextValue(oDiscCalculator.discWidthMin, iWidthMin);
            setTextValue(oDiscCalculator.discWidthMax, iWidthMax);
        }
    }

    function setTextValue(nNode, sValue) {
        sValue = String(sValue);
        sValue = sValue.replace(/\./, ',');
        nNode.innerHTML = sValue;
    }

    function isClass(nNode, sClassName) {
        return (nNode.className.indexOf(sClassName) >= 0);
    }
 
    function addClass(nNode, sClassName) {
        if (nNode.className) {
            var aClass = nNode.className.split(' ');
            for (var i in aClass) {
                if (sClassName == aClass[i]) {
                    sClassName = '';
                }
            }
            if (sClassName) {
                aClass.push(sClassName);
            }
            nNode.className = aClass.join(' ');
        } else {
            nNode.className = sClassName;
        }
    }

    function removeClass(nNode, sClassName) {
        if (nNode.className) {
            var aClass = nNode.className.split(' ');
            for (var i in aClass) {
                if (sClassName == aClass[i]) {
                    aClass.splice(i, 1);
                    break;
                }
            }
            nNode.className = aClass.join(' ');
        }
    }

    function doPreload(sImg) {
        var oPreload = new Image();
        oPreload.src = sImg;
    }
})();