Simple Speedometer - lua scripting

You can find some syntax examples in the wiki: http://wiki.beamng.com/Lua . If you have any programming skills in some language that you can easily learn/understand lua.

P.S. I did a simple speedometer by changing the game files: adding another debug screen. But this is not the best solution, i think.
Need to wait documentation in which will be described how correctly writing own scripts for BeamNG Sandbox.
P.S. This is a simple speedometer:
1. Open lua\vehicle\bdebug.lua , found

Code:

local modeCount = 12

and change 12 to 13:

Code:

local modeCount = 13

In this file after

Code:

elseif M.mode == 12 then
        txt = "Lua Performance Debug"

add:

Code:

elseif M.mode == 13 then
        txt = "Speedometer"

2. Open lua\vehicle\canvas.lua and after

Code:

self:drawLuaPerf(dt)

add

Code:

elseif modeNumber == 13 then
        self:showSM(dt)

Then after end of the DrawingCanvas:update function add

Code:

function DrawingCanvas:showSM(dt)
    self.ct:clear()

    if not self.debugCounter then
        self.debugCounter = 0
    end

    self.debugCounter = self.debugCounter + 1
    if self.debugCounter > 32000 then self.debugCounter = 0 end

    local airspeed = obj:getVelocity():length()
    local txt = "airspeed: " .. math.floor(airspeed * 3.6) .. " km/h, " .. math.floor(airspeed * 2.23694) .. " mp/h"
    self.c:drawText(txt, string.len(txt), 5, 70, self.paints.fontLeft)
end

After these changes will be added 13 debug screen, where will be only a vehicle speed (and some debug info).