to build on what Mr. Gray tells us:
This is a poem of the pre-jazz era western world; electricity was more than
rare and it certainly wasn't used for heating. Cars were also rare (and so you
are correct in assuming they aren't picking up petrol on the pavement).
The image you can think of is twofold. If you have seen _Monty Python's The
Search For the Holy Grail_, you can think of the silly characters who are
collecting mud (and imagine them gathering peat or wood or something)--
Or you can think of bent-back peasants picking up coal scraps from the lots
where the coal trucks (or carriages) parked. When the coal trucks moved,
precious coal fell out. This is covered in the movie _My Left Foot_ when the poor
Irish would go into the streets as the truck passed and collect the fallen
coal they could not afford.
My guess is that Eliot was thinking of the latter.
Pax et Lux,