Since decent 12V to USB 5V converters are readily available, I would put one in between car battery and Pico. Reason is that - depending on what you drive with the Pico pins and what else you 'hang onto the Pico's .3V output - the Pico onboard voltage regulator may heat up... after all, with 14.4 Volt - typical for a running engine - and a 250mA current you convert 3.7 Watts into heat. If you let the 12V to USB 5V converter do the heavy lifting ('destroying' power / converting power to heat - up to 3.275 Watts), it will only be 0.425 Watts left for the regulator of the Espruino to 'dispose'.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Since decent 12V to USB 5V converters are readily available, I would put one in between car battery and Pico. Reason is that - depending on what you drive with the Pico pins and what else you 'hang onto the Pico's .3V output - the Pico onboard voltage regulator may heat up... after all, with 14.4 Volt - typical for a running engine - and a 250mA current you convert 3.7 Watts into heat. If you let the 12V to USB 5V converter do the heavy lifting ('destroying' power / converting power to heat - up to 3.275 Watts), it will only be 0.425 Watts left for the regulator of the Espruino to 'dispose'.