Well that depends on the load on the battery doesnt it - the radio on ? headlights ? running the air conditioning ? you need to measure the current drains of all the essential electronics of the car - fuel pump, guages, ignition system, injectors, engine computer, etc. everything that would be active during your run. then the battery would have to supply that for as long as it was able. the battery condition is important too since a weak battery would not last as long as a new one. so lets think about a freshly charged healthy battery which has just taken on its load alone. Car battery with 850 cold cranking amps. thats 850 amps for one minute or one amp for 850 minutes ( sort of ). so now how much are we drawing ? lets say the essentials are drawing 25 amps. ok so 850/ 25 is 34 minutes. More if the battery is hot, less if the battery is cold. so now how fast were you going ? the ignition draws current related to how many sparks it has to generate per second, the fuel pump pumps more when you use more fuel. so maybe 25 amps is at 30 MPH. so at 34 minutes you might get about 15 miles distance. because the ignition will drop out at some voltage before the battery is dead, and the battery voltage will slowly decrease from 13.8 full charge volts to 10 volts dead and where does the ignition drop out ? maybe 11 maybe 10.5 ....... so it depends on a lot of things. 15 miles ? thats just a "what if" figure based on a couple of guesses we made.