You have much more to worry about than the distance people will drive. Petsmart, Petco, ect have "Walmartized" the pet buying industry and open their locations to drive out mom and pop shops on purpose. The city I live in is mid to large sized. 10 years ago we had 6-7 mom and pop shops which the majority of them were great. 2 petsmart, 2 incredipet and 1 pet supermarket later and all but 2 of the mom and pop shops are gone. Where do you think all of these places opened up? All of them opened within 2 miles of an existing privately owned pet shop and all of them had better store fronts, marketing, ect.
I routinely drive 70+ miles to buy fish and supplies from someplace other than a Petwalmart. However I would drive 100+ miles to see one burn down.
Petsmart is de debil.
Petsmart proponents speak up but I have never been in one where the employees knew a apple snail from a guppy without the picture to help and if you have a question about your tank? Forgetaboutit.