Keep in mind that an engine with 312K on the clock is likely to have dropped a few points in the compression department. You cannot achieve fuel economy in the 26-27 range with such an engine. However, with checking on all the little details, as noted above in previous posts, you should be able to get it up to 23-24 or so.

Also, note that speeds that reach or exceed 70 mph will kill any chances of getting any serious mileage improvements. If you are serious about fuel economy, spend another 4 minutes for the commute and drive it at 65 mph.

One additional item, not posted above, but important for fuel economy, is to disconnect and plug the EGR.