"Mountain West states Wyoming, Idaho and Montana followed Utah in having the nation's lowest gas prices. All four states have two things in common: close access to fuel from Wyoming and Canada that reduces transportation costs and high elevations that make it possible to use lower-grade gas. Regular gas is typically rated 87 octane in much of the country, but in Utah it's typically sold as 85 octane"
State taxes have very little to do with it...the lower octane sold as regular makes most of the difference, coupled with shorter transportation for both the crude, as well as the finished product.