Left in the desert question

When the mechanic leaves Tanner to die in the desert/firing range, he wakes up and sees the mechanic driving off, but begins walking through the desert/firing range in the opposite direction.

If the mechanic is driving back to town, why didn't Tanner walk back in that direction? It isn't until later that he comes across the oasis and calls for a cease-fire by setting the oasis on fire so he wouldn't be walking to the oasis, would he?