Motor fuel and special fuel are exempt from sales tax when used on the highway or in watercraft or aircraft if the fuel tax has been paid and no refund of the fuel tax has been allowed. When a fuel tax refund is allowed, the sales tax is deducted from the refund unless it is used for a purpose that is exempt from sales tax. In some instances, the amount of sales tax which would be due may exceed the amount of fuel tax refund allowed. This can happen because the fuel tax rate is a flat amount per gallon and is not impacted by the price of the fuel. On the other hand, sales tax is based on the price paid for the fuel, so as the price goes up, the sales tax also goes up. If this situation occurs, taxpayers may elect to not claim the fuel tax refund, since if they did, they would owe the difference between the sales tax and the fuel tax refund. Dyed diesel would normally be subject to sales tax, unless the purchaser qualifies for a sales tax exemption.