Don't buy diesel along major highways where semi trucks frequent unless you don't mind to paying more. The price is usually inflated considerably because the fuel stations know that truck drivers do not want to drive away from the highways to look for cheaper fuel especially when it is the company they drive for that is paying for it. Try and find a fuel station that sells diesel a few miles away from the highway which should be very easy with a smart phone. You should see a huge difference in price. Although gas is usually cheaper at these station because they are trying to compete the gaser vehicles business.