Legally, they should replace the tires--when you purchase a truck, it is with the reasonable and justified expectation that it has the correct tires on it (unless you are specifically told otherwise and choose to buy it anyway). If the truck does not have the right tires, that would be a breach of one or more of the following: the agreement of sale; the covenant of good faith and fair dealing; or an implied warranty of fitness for the truck's intended purpose. It could also be fraud, if the dealer knew of the wrong tires and concealed that fact.Practically, if they won't voluntarily pay, you'd need to sue; to win the suit, you'd have to show that the tires were the ones on the truck when purchased. It may not be worth the expense and time of a suit, given that winning is not guaranteed, since the burden of proof would be on you, as the one suing, to make your case.