I have a new vehicle and an old vehicle. My wife commutes 18 miles through the city (no freeway). I commute 66 miles over mostly freeway.
In terms of vehicle longevity, which car should take the long freeway commute and which should take the short city commute?
I would drive for a month and then switch for a month…compare comfort and gas mileage…you decide.
If they both run well, I’d drive the older, high mileage car on the long commute. at 33 miles daily, you will put 16,000 miles on the car commuting; over 32,000 miles yearly if it’s one-way mileage. There will be higher depreciation on the new car due to the mileage. The older car already has depreciated significantly, and won’t depreciate nearly as much in dollar terms (as opposed to percentage).
Agree with jt; wear out the old one since it has a lot of life left in it. I’m assuming that both are equally comfortable, and you would not be making personal sacrifices.
Interstate driving is much safer than city driving. I’d use the one with the most airbags, ABS, ESP, etc. around town.
Twotone
18 miles through the city is a tough commute. 66 miles on the freeway is a piece of cake. Switch back and forth occasionally to even the wear. Don’t condemn either vehicle to the city commute.
Letting your wife drive the new car most of the time is probably the best idea.
What gas mileage do they get? I’d put more miles on the most fuel efficient car.
Thanks all. I’m mainly just concerned with keeping the cars running longer. Depreciation is something I hadn’t considered, but that’s useful. Safety is nearly the same in both cars, though the newer one has side curtain airbags while the other does not. Gas mileage is virtually the same. No one mentioned my main requirement: the stereo system. Fortunately I made the decision to purchase both vehicles, so they’re equally well equipped in this regard 
It sounds like I’ll take my '01 Outback VDC with 180K on the 132 mile/day commute and the '08 Sienna Limited w/ 30K will do the 36 mile/day city route with my wife. (Don’t worry - if the job goes permanent we’re moving to the other side of town ASAP).
As a general rule, I’d take the vehicle most suited for the task at hand, regardless of mileage. If they are close in performance and one trip through the city is harder on a car, I’d alternate them. If you keep both cars till they drop, the depreciation argument means little to me.
Assuming you’re driving an old car with long service intervals and good fuel economy, I am guessing that the old one will cost less on the longer commute than the new one. The miles will accelerate the depreciation of the newer car, which is going to be your biggest cost. It’ll probably cost more than the maintenance to keep the old car going. (Even if the old car is something like a Dodge Caravan or some other money pit, that’s probably the case.)
Otherwise, I don’t think you can really say what will live longer. It depends on more than how old each vehicle is.
Once an old, running vehicle gets down around the 1000 dollar range, as long as it remains in roughly the same condition, it’s pretty much done depreciating.