That would be an interesting exercise, but I'm not sure it would work. Take those values from OBD, do the math and display the results. However, each gear works for a wide range of rpms, and this would not take acceleration into account. For instance @ 35mph and 1800 rpm the Sequoia could be slowing to an eventual stop in 5th or 6th gear or accelerating hard in 1st gear, so you'd have to take into account the acceleration inclination too. That is possible because of the wide range of torque available, that allows a big engine to work at such low speed/rpms in high gear. The Civic Sized, 414HP/400ftLb IS-F with 8 gears is even worse, you can launch the car in 3rd...and it can pull from 8th gear in manual mode from idle at 35mph no problem.
So I don't know, maybe this can work, just not sure.