Since nobody mentioned it, have you checked to make sure the tires are inflated correctly and the wheels aligned? I went from low 20 mpg to around 25 mpg just inflating the tires to the correct psi and setting the correct toe.
I doubt it has anything to do with the timing belt or timing. If it were off a tooth, there would be a noticeable decrease in power and it wouldn't get anywhere near 24 mpg.