Yep, it is the fuel taxes. I run commercial and have to track the fuel tax, state by state. At the pump, Illinois has a relatively high price, but when you factor out the tax, Illinois fuel is cheaper than the surrounding states even after you factor out the other state's fuel tax. I paid ballpark $15,000 last year just in fuel tax.
Whether the cost makes that much difference is relative. When I purchased my '06 Jeep Liberty Diesel, many shook their heads, especially when diesel prices soared in '08, and wondered why I would run a diesel. Well, since the average 3.7 gasser Liberty is lucky to get 21 mpg for a road trip, my diesel version consistently gets 30+ on a road trip. Even with city and rural farm road driving, my diesel Liberty mpg is always higher than gasser version road trip mpg. So, diesel may be higher than gas, but it would have to have a lot larger spread before having a diesel would be, on equal, the same cost per mile as with a gasser. I'll keep the diesel.