How can you run faster in heavier shoes?

I have noticed that since changing shoes on my easy/GA runs from the Brooks Kinvara I had been using this summer to a heavier (and much older pair of) Brooks Ghost shoes this week, running times for my usual 8 mile run have improved by 20 seconds/mile. Much lower heart rate, too. Dew points in the AM haven't been that different.

What gives?

Too much flexibility in the Kinvara's? The effect of one 26.2 mile training run 10 days ago? Mental?

I think the answer is that there is really little correlation between weight of shoes and running paces, especially in training runs. There may be some in races but I think you are looking at the wrong variable in why your paces have improved. Faster paces plus much lower heart rate? That's typically the result of cooler temps, that same thing happens to me usually in the fall when temps drop pretty good.

My bet is that your fitness is continuing to improve.