I know this is an older thread. But I'm on my second set of Ecopia 422 (maybe third...since I wasn't the original owner and I don't remember what came with the car at 20k miles). I live in socal and have always gotten around low 40s MPG with a heavy mix of stop and go street traffic and lots of highway driving. I don't have a super heavy foot but I'm not the lightest foot either.
Tire manufacturers say LLR tires save 1 to 4% in mpg. For perspective that's about .5 to 2 mpg on a 50 mpg vehicle. Wet & dry stopping distances, handling crispness, ride quality may all be more important over the life of tire. It is for me anyway. Price will have to be balanced against service life because a cheaper tire set worn out in 30,000 miles may well cost more than a premium tire set that goes 60,000 and handles better & safer every one of those miles. It's hard to choose if you don't know much about tires yet but comparing the Uniform Tire Quality Grade for the tires can help. UTQG gives estimated service life (100 to 800), a grade for wet stopping distance (C to AA), and ability to dissipate heat (C to A). A or AA is best and a 400 grade tire lasts twice as long as a 200. Big box stores like Costco and Walmart have the leverage to get better pricing from the manufacturers on tires with premium UTQG grades.
I have had good luck with the Michelin Defenders, though I got mine installed at Pep Boys (this was a few years ago, as I was not a Costco member at the time). Pep Boys has a 10% Senior Discount that stacks with their sales price, and can further discount the road hazard warranty if you opt to buy it. Much better than the Walmart Douglas tires that were on there before.
