How far do you need to go? I've always thought of earthquakes as fairly localized events: moving a dozen miles in stage 1 evac - in the right direction! - should get you out of the path of initial fires started by the earthquake, and another dozen miles out of any later after-affect fires, etc, and from there it's mainly a matter of getting far enough away to be get able to get supplies (gas, food, lodging) normally & wait for the the OK to return.

A fast truck is useless: you want a diesel that is efficient at speeds under 5 mph, and at idle, because that's all you'll achieve when a million people are also evacuating on the same road.

There won't be any fuel left at the gas station by the time you get there so you go as far as you let your tank run down before the earthquake. A big tank you let run down to the last 5 gallons goes as far as a small tank run down to the last five gallons...

A good set of up-to-date road maps, with several pre-selected routes based on which direction you need to evac, and "practicing" each along with candidate detours, is probably more important than which vehicle. Make sure it has a good radio.