The mileage is worthless. It's nice to have outside temp, that's all I use mine for
I've yet to hear of a brand of truck that those things are true to what mileage you get if you do the math. Just set your trip meter and see how many gallons it takes to top off the next time. That's the best average you can get.
Not sure if the wiring is present, but you could look for a truck at a salvage yard and give it a try
2004 Crew Cab 4x4, 305 5.9L, K&N intake system, Banks exhaust system. Hankook MT 265/70-17
First diesel, so still learning