Besides driving slower or not at all there is nothing we can do. I will assume that the real goal here is to save money rather than just get better mileage. And if saving money is really the goal then buying new tires, adding manual hubs, or really spending any money towards mileage is not at all cost-effective. It's kind of like spending 25K on a Prius so you can save money. Sure, you get great mileage and it only cost you 25K. If you sit down and figure out how long it will take to see any payback you will see that it is usually not worth the cost. The exception would be if you needed tires, or a new car, or whatever you were considering buying anyway and you bought with mileage in mind.
The best alternative and one that we all can take advantage of is making our own fuel. I know a lot of guys that do that and they make bio for less than $1. 00 a gallon, including methonal and water and electricity. It is work but if you drive a lot you can save big money. The gain from inflating your tires to the maximum would be neglible and then you just wear out your tires faster, which again, costs money to replace.