After the head job, the gas consumption increased to 15-16.5 mpg in three camping trips. The compression is good, spark plugs, rotor, timing, air cleaner, wire connection, vacuum are all fine. I also took the catalytic converter out and found the ceramic materials inside are clean and in excellent condition, the O2 sensor was replace about 6,500 miles ago and still nicely in place, connection no problem. On the freeway, I notice the rpm at 65 mph is about 3,900, at 70 mph is way over 4,000. I remember before the head rebult, the rpm was 3,800 at 65 mph, and slightly over 4,000 rpm at 70 mph. I wonder what is this mean? I put 2 new tires at the front, would that be the reason for that? The multivan drive well, smooth, but doesn't seem to perform well at higher speed. The gas mileage for the first tank of gas after rebuild was 15.3 mpg, the second tank was 16.5. Is it necessary for the engine to break-in after the head replacement? I used two new AMC heads from BusDepot, and replaced
 all 8 hydraulic lifters.

Leo, 1991 multivan