I recently got a set of new tires(Falken Sincera Touring). Previously I had the stock Goodyear. Right after I got these tires, I went on a 600 mile trip, 90% highway so I was averaging 45MPG. Ever since then, I've been averaging 51 on screen and this is 4 tanks after the trip. I used to get 57 on screen. Do tires really make that big of a difference? I bought these tires for its 720 tread wear rating. But doesn't seem to be saving me anything if this is the reason for the 6mpg difference!!!
I'm figuring my new tires are costing me 10%. Most of the mileage on the new tires is cross country Intersate driving which would have been around 50mpg on the original tires. Looks like you are taking a 10% hit as well.
