Why are you getting into your car then using your phone? You shouldn't be using it while in your car anyways. That said, sounds like either your car is parked several hundred feet from your house, or your router sucks. My AirPort Extreme is accessible with full connection from my driveway and its 50 yards from the house. This isn't something easily fixable by apple without killing battery life because the phone would have to constantly be evaluating cellular vs wifi strength. The solution is upgrade your router for a hundred dollars.
You are not understanding the underlying problem. ANY wireless access point will have a region where you can get reception but the bandwidth that is able to be provided will be very marginal. Yes, even your precious AirPort Extreme.