Do I have to pay taxes on money that is reimbursed to me on my paycheck

by John
( Washington State)

Hello, I drive a company vehicle to and from work and my employer covers all fuel costs. He provides this money to me in the form of a non-taxed reimbursement on my paycheck. However I just realized that this money, while taxes are not being taken out of it, is going into my net pay. Am I going to have to pay taxes on this? Its not income for me, but there will be a large amount of money in my net pay at the end of the year that won't match up with the amount of taxes taken out. I give my boss all my gas receipts.