Tires: Tire weight vs. Mileage, gas mileage, flat tires

Question
Last summer I bought a used 2004 Sienna LE AWD.  It came with Bridgestone Run Flat Tires.  While my mechanic had a tire off to inspect the brakes he noted that it weighed much more than a standard tire.

I did some research today on the Bridgestone web site, and it seems like a normal tire weighs about 25-27 lbs, while the RFTs come in at 38 lbs.

What I'm trying to figure out is how much the extra 50 lbs of tire weight on the van affects my mileage, and whether it is worth it to switch to regular all-seasons before my current tires wear out.

Answer
Any weight effects gas mileage, but 50 lbs is hard to see.

The benefit that comes with run flat, being able to drive on a flat tire is worth it.

The tires weight more because of the extra steel in the tire that makes it able to be driven on while flat.

John