Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
Fuel tax
A fuel tax (also known as a petrol, gasoline or gas tax, or as a fuel duty) is an excise tax imposed on the sale of fuel. In most countries the fuel tax is imposed on fuels which are intended for transportation. Fuels used to power agricultural vehicles, and/or home heating oil which is similar to diesel are taxed at a different, usually lower rate. The fuel tax receipts are often dedicated or hypothecated to transportation projects so that the fuel tax is considered by many a user fee. In other countries, the fuel tax is a source of general revenue. Sometimes, the fuel tax is used as an ecotax, to promote ecological sustainability. Fuel taxes are often considered regressive taxes.