GPS modul. How to get speed and put it in variable?

In general, going on the path 1 with yours help, I wrote such code, but there are some strange mistakes = (such as 45 km / h at rest). I tested it in my car. Visually it coincides with the speedometer in my smartphone =)

//********* 
Serial3.setup (115200, {rx: P0, tx: P1});

var R = 6371e3; //
var Led= require('@amperka/led').connect(A1);

var lastData; // 
var lastData2; //
var lastLon;  //  
var lastLat; //  
var LastTime;
var toRad=Math.PI/180;
var i=0;


var gps = require ("GPS"). connect (Serial3, function (data) {
  
  if (lastData,i<50) {
   // do stuff here - eg. the difference between the two readings
  //****** distance calc  
    var Lon=data.lon*toRad; // lon в  t1
    var Lat=data.lat*toRad; //lat в  t1
    var diffLon=Math.abs(Lon-lastLon*toRad);
    var diffLat= Math.abs(Lat-lastLat*toRad);
        var a= Math.sin(diffLat/2) * Math.sin(diffLat/2) +
        Math.cos(Lat) * Math.cos(lastLat*toRad) *
        Math.sin(diffLon/2) * Math.sin(diffLon/2);
    
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
var distansP = R * c;
//****** time diff
var tTime= data.time.split(':'); // time array
tTime=tTime[2];
var diffTime=tTime-LastTime;
    //****** speed
    var speed1= distansP/diffTime; // m/s
    var speed2= speed1*3.6; // km/h 
    
    if(speed2>60){
      
      Led.turnOn();
    }
    else{Led.turnOff();}
 console.log(speed2,speed1);
 //console.log ('1',Math.sin(diffLat/2),diffLat/2,'2',M­ath.cos(Lat),  '3',Math.cos(lastLat*toRad),lastLat*toRa­d,'4',Math.sin(diffLon/2),'5',Math.sin(d­iffLon/2),'6',Math.sqrt(a),a,'7',Math.sq­rt(1-a),'8',diffTime ); Debugging
 
  }
  
lastData=data;
lastLon=data.lon; // in degrees

lastLat=data.lat; //in degrees


LastTime=  data.time.split(':');
LastTime= LastTime[2];  
  
//  lastData = data.time.split(':');
//lastData2 = lastData[2];
//  lastLon= data.lon;
  i=i+1;
 
});