North of Chumpon, about half way between Phuket and Bangkok, is a very large service station which is a stopping off point for the buses as they make their way north.
Around this area, the famous regional produce is Se-La-Pow, the steamed bun filled with salted eggs, pork, red seasoned pork, beans and even sweet custards. There's lots of varieties of steamed bun, so you won't be spoilt for choice.
They're very cheap (mine were 7 baht a bun) and because of the high sales volume, very fresh. Steamed buns are perfect for road-trip food, they are ready to eat, and not messy, so they don't mess up your car.
On a trip from Phuket to Bangkok, Chumpon as about half way, which is the perfect snack time too. There's several of the larger service stations at Chumpon, the long distances coaches stop at these, to provide the free included meal.