In Washington state, Attempting to Elude a police vehicle is a felony. RCW 46.61.024. That means that the maximum penalty is five years in prison and/or $10,000 fine. RCW 9A.20.021.
In practice, a first-time offender with no prior record is usually looking at 30-90 days in the county jail.
As to using “I didn’t know it was a cop” as a defense, Washington requires that the pursuing vehicle use lights and siren and the officer has to be in uniform. The crime also requires reckless driving, so simply refusing to pull over because you aren’t sure if you’re being followed by a real cop (I wouldn’t pull over for an unmarked vehicle - I’d keep driving until I could pull over where there are lots of people or until a marked police car showed up), it isn’t eluding.
I feel very strongly that the penalties need to be increased for this crime. Due to liability (and dawning good sense), many agencies are restricting the use of pursuits. If the circumstances do justify a pursuit, then the punishment should be severe.
I have a favorite story about pursuits and the difference in state laws. Many years ago, a driver drove away from an Idaho state trooper and it turned into a pursuit. The idiot driver ran across the state line into Washington, believing that the Idaho trooper couldn’t cross state lines (we have pacts allowing this with the states that we border). A Washington trooper joined the pursuit at the state line, and the driver finally pulled over a few miles into Washington. Well, eluding a police officer was a misdemeanor at the time in Idaho, but a felony in Washington. So not only didn’t crossing the state line help, it made the crime considerably worse.