What you need:
  • an image available online, showing the blueprint of the original vehicle (there's plenty of blueprints e.g. here)
  • the URL address of that image (can be obtained by right-clicking the image you found online and choosing "Copy image link")
  • known dimension - one dimension of your model that you know (e.g. wheel diameter or track's width)
Need help? Try watching this:

complete video tutorial
Paste & load URL address of the image file (NOT of a website that includes this image, but of the image itself - it usually ends with .jpg or .png):
Or upload an image from your device:
Draw a dimension you know on the image below (e.g. wheel's diameter), then enter how many studs it corresponds to in your project:
Note: the width or height is always taken from the first measurement, no matter how many you draw and even if you clear them all. It lets you change the dimension above and get updated results without starting anew.
Calculate the scale:
If you know the real counterpart of the dimension you entered above, enter it here to calculate what scale your model is in:
When activated, click any two points on the image below. A line will connect them and the line's length and angles will be displayed below. Click anywhere again and a new line will be added connecting the end of the previous line with the new point you've just addeds. Length and angles values will be updated to show that new line. You can chain up any number of lines this way. Deactivate to return to standard drawing.
Point #1: unknown [ reset ]
Point #2: unknown [ reset ]

Distance: unknown
Angle: unknown
Transparency level for inactive lines:

Transparency level for inactive labels:
Canvas width (%):
Canvas height (px):
Line / digits color:
Labels color:
Clearing the measurements:
Only clears protractor lines while protractor is active
What next?
The easiest way to save the resulting image is to use the Print Screen button and then to paste the image into some image manipulation program - or even easier, paste it directly into Pasteboard.