In hindsight I guess I'm becoming pessimistic. I didn't think so but I realized when somebody tells me something, like "These tires will get 10% better fuel mileage" my first thought is "Bull crap".
I actually though was proven wrong, well at least it looks that way (See? pessimism....).
I bought new tires for my VW Jetta TDI and was going to go with the stock ones. The sales guy at the tire place though recommended these other tires called "Ecopia" because they have better low rolling resistance. They were about $70 more than I planned to pay, which was probably $50 less than I really needed to pay.
At first I wasn't going to but they come with a 30 day return if you don't like them. (I'm sure there's a catch though). I happened to read a review and a user claimed 1-2 MPG better and with the 50k miles a year I drive that was enough to sell me.
I put them on and this morning when I drove to work I actually got 53.4 mpg over the 58 miles. I actually have done better than that on a very rare occasion (once in 3 years) but I also hit a number of red lights, and stopped at the bank ATM so was very happy with that. My average was 45.9 and usually on the ride to work if I hit 49-50 I'm pretty happy.
The one thing I did notice, that I need to get used to, they coast longer, so I need to let off of the gas sooner than I'm used to. This means I should get even better mileage as I tune my driving to the new tires.
The morale of the story though is try it before you buy it (or need to keep it). This isn't just for tires, but for everything. Many vendors claim a lot of things, but before that PO goes through. make sure they show you or at least make sure you have it in writing that you can return it for free if it doesn't meet your needs....
UPDATE on MPG
So after a week my average MPG has been 52.2MPG. TO be fair I haven't been anywhere but back and forth to work (no downtown Boston driving or highway) but this used to be around 49.5 or so and is now over 52 so I'm pretty happy. Also I looked at their calculator and think it is pretty accurate (for me anyway) http://www.bridgestonetire.com/fuel-calculator if you want to try it.