Are the new front tires larger than th the rears? Th heads did not change the RPM to speed ratio. Are you packing stuff on the roof or using the AC? Maybe you have a brake that is dragging. Is this an automatic or 4 speed. An automatic with the AC on and stuff on the roof will not do any better at 70 mph. Just because the O2 sensor is new does not mean it is good. Test it to make sure it is working properly. That may explain the poor hi sped performance.

Dennis

----- Original Message ----- From: Leo Lee <leo0365@SBCGLOBAL.NET> Date: Monday, April 18, 2005 8:16 am Subject: Bad has milleage

> 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 >