An easy and low processor load way to read your wheel encoders, or any quad encoder for that matter.
You will need a quad encoder of some type with channels A and B
Two flip flops, which can be handily had in one package.
This is a the circuit :-
When a change of state is received by the flip flops they will translate this to a pulse on either the forward or reverse output pin. This can be counted by a microprocessor with an interrupt on a pin for each directions, then either add or remove one from the distance count to calculate wheel rotation.