How about a few different tones? There could be options for a zero power tone while in a drive mode, a tone for the middle portion of power, and a more annoying tone for the top portion of power consumption. The first portion would be silent, except for a user programmable threshold in kW.
Each tone can be individually turned on or off, and the programmable threshold could be set for anywhere from 2-50 kW.
Tigger, I have a Galaxy S Captivate, a Galaxy S3, and an Acer Iconia A500 tablet. All three run CaniOn well, especially with the Cockpit view turned off. I'd say any upper end phone from the last 3 years should work well for you. There is also the custom ROM route if you need to boost the speed of the phone.
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015
"Photon" - 2017 Bolt EV LT in Orange with QC - 7/31/2017
Solar-powered since 10/10/2013