I know that to be true. I would say the weight savings with the new wheel and tire combo on our Land Cruiser is 35 lbs, or 7 lbs per.
The point I was making is that with 400 miles on the clock and 12 mpg displayed, the amount of that mpg attributable solely to tires isn't going to be as massive as OP thinks. He is going from one tire that is relatively heavy to another that is in the same ball park as he thinks the tires on the vehicle as delivered to him are the main reason for achieving 12 mpg over a short distance.
Edit- that 35 lb weight saving is more than offset for me five days a week by lugging all the crap I use for work around, but that gear is not unsprung weight,