How to enter "engineering test mode" in a 2009 Mustang GT.?
Hold down Reset and turn the key to ON or start your car. When it says "engineering test mode" release the button. It should start in gauge test and do the gauge sweep. After that, keep pushing the Reset button a few times (maybe 20?) until you get to spedo mode. If you miss it, just keep pushing the reset button until you get to it again. It should be in there somewhere, just keep pushing the button. You can start your car and it should stay in ETM. To exit ETM, hold down reset for a few seconds or just turn the key off. If it doesn't work as advertised then your cluster doesn't support it.
One note is the fuel status--it always shows twice your fuel level. So 150% would be 3/4 tank.