New car mileage

Dad had a 86 Lincoln continental. Fuel injected 302. Averaged 24-26. On the hi way. He had some shoulder wear on his tires and was asking for advise on replacement as he was going on a 2000 mile trip. The center of the tire had a lot of tread. I recommended running them over inflated For the trip and replace them after the trip. He got 31 miles to the gallon on that trip. Must have been like bicycle tires going across the desert.