Several comics, cartoons, and even a couple games shows they actually live in large and modern well populated cities. Robotinik is not the only one building cities there.

I've slept since then, but I could have sworn in one of the games Tails mentions his plane is powered by one of the emeralds.

Can a biplane fly on ethanol? Stills can be made out of almost any thing.

Since Sonic is so fast can’t they just make a quick run to the nearest gas station even if it is really far away?

We getting porcupines and hedgehogs mixed up?

Clearly his plane runs on biofuel.

BOW CHICKA BOW WOW

It's weird they gave Tails a house/workshop in a game, but insist that Sonic actually does just live in the woods and runs around from place to place.