There’s a couple things going on here.
First, your state lemon law sucks. Nobody’s state lemon law actually applies to these situations, but Nissan has decided they’re going to use whatever formula is specified in each state’s lemon law for calculating valuation amounts. That’s both good and bad. If you were to bring a legal action in your state, and a buyback were to be awarded, I imagine a formula that’s already well enshrined in law would be used so that’s why Nissan wants to make sure they’re offering what the law would require. Problem is, the arbitration folks have been claiming to people that they have to go by the lemon law, and can’t actually offer any more than what the lemon law authorizes. That is complete and utter horseshit.
In your particular case, the Colorado lemon law doesn’t provide any formula for “reasonable allowance“ of usage, and also doesn’t provide any sort of cap on the amount. So, you’re not starting out on a great foot.
The two most common formulas that I’ve seen in state lemon laws are either a fixed amount per mile driven (like $.10 or $.12), or a fraction based on how much of the car is used, with a numerator of miles driven and a denominator of something artificially low for the present time, like 100k. In that method, driving 67,000 miles would mean your car is 2/3 “used up“ and so they are responsible for only giving you 1/3 it’s original value.
You’ve got a couple of points of leverage to go back to them with. First thing should be that you ask them for an explanation of how they arrived at that usage number. I am suspecting they’re using miles driven out of an expected lifespan of 100 K miles, but before you start argue with them that they shouldn’t use such a dumb calculation, you should be sure that that’s what they’re actually doing.
Since there isn’t a formula specified in Colorado state law, I would next insist that they use the mileage you had when the warranty claim was first made (59K?), and then try to convince them to use something way more reasonable like $.10 a mile. The law’s not on your side on that one, but it doesn’t support them really either, so that seems to be a point of negotiation.
The other thing that works against you is that rebate. If that was some actual
point of sale rebate from Nissan or something, then they’re probably right to exclude that from your calculation of your total outlay. However, if that’s a tax credit you had to file for or something, that’s bogus.