KiwiME: I've considered leaving a second phone in the car, more for a permanent but hidden mounting position rather than the moment it takes to plug in. But don't want to pay another $19 a month for a SIM.
2degrees has free data sharing. If you have a post-pay plan you can share your plan data with up to 5 other pre-paid sim cards. I think it is designed for people with Ipad Airs/tablets etc. I have a phone that lives in my car and it costs me nothing per month for the data it uses.
Edit: also ideal for GPS trackers/security systems etc.
That's a good thing to know, tripper1000...FredNZ's idea of having a cheap phone permanently in the car might be a good idea now that you've told me that 2Degrees (who my cellphone account is with) data share.
I could indeed get a prepay SIM and only use it for instant NAV (plus have all my music on it too)...something to consider.
Meanwhile, after charging to 100% this morning with my current city-driving usage at 10.7kWh/100km, my range was showing 609km this morning...that's pretty impressive, eh?