Beat way is for you to run your car outta fuel with a gallon in the boot, put that in and see if you travel more than 4 miles! Clearly you will unless you've got a crazy displacement. Odds are on that you'll do more than 18....... The computer is fairly crap. I've got mine to read 48 mpg if I drive carefully but again that's wrong. I asked nick at rtec when I got my spec 2 and he said the OBC reading goes outta the window with the map and turbo etc