We need some further information about exactly what you are doing.
The Picaxe and other similar chips MUST still be run at 5Vdc so you will need a 7805 somewhere in the circuit for them.
The 7805 can operate with an input voltage up to around 30Vdc but heat dissipation needs to be taken into account depending on voltage drop across the regulator and current draw.
In the same line as the 7805, there are also 7812 voltage regulators for 12Vdc output.
But this needs around 14 to 15V to operate as must take into account the drop out voltage of the regulator.
Since it is for a car, there will/may be issues with voltage fluctuations and interference/EMI to take into consideration. tehrefore may be you need far more than just a 7812 regulator
So please post you proposed circuit and we can advise better what you may do.
I'm having trouble understanding your question. Do you want to know what the maximum input voltage is for a 7805? According to the datasheet you can supply 30Vdc.
So you can use a 7805 directly from a car battery. I would use a 16V Zener diode to suppress any voltage spikes and a capacitor before and after the regulator to keep the voltage clean. See the page 21 of the first picaxe manual for suggested values.
EDIT: sorry if i repeat Wesaust55, we were typing at the same time