Note that idioms are expressions (words or sentences) that are used figuratively to communicate ideas in English.
For example, if in a passage one character John tells another Peter that 'I'm all ears, the word ear mentioned by John is an idiomatic expression that means he has given the other character his full attention. If you were to take it literally it would mean he is saying he is an ear.
"Wheels" refers to the wheels of the car beginning to turn as Milo thinks. It also refers to the "wheels" of Milo's mind turning after not doing much thinking for a long time.
