Michael, there are lots of data logging systems with their own sensors and many have displays. Do you HAVE to use an iPhone? That display is pretty small in a vehicle.
On my bike I use the AimSports digital dash that also logs many sensors such as RPM, speed, several temps, several pressures, voltage, and more. There are lots of competitors to Aim too, such as Innovate. Have you considered that route?
Your neighbor in Puyallup.
Don