Math on having a second vehicle

I had a similar scenario. I got a 1997 Saturn commuter/ run around beater. Gas mileage is in the high 30s. The Excrusion stays home and is used for towing, trips and family outings. As a result it only has 86K on it and is a 2001. The way I figured it I would rack up the miles on the beater and extend the life of the Ex. It also delayed me having to pay to replace Ex. The beaters I buy are under 2500 bucks. They are high mileage but reliable. The saturn has 165K on it. We have a second beater and its a 2001 Sentra that has 136K on it. My wife drives it on crappy/ snowy days and leaves her Mustang at home. Insurance is next to nothing on them and they cut us a break on the Ex and the Mustang because we keep the yearly mileage down on them.