Fuel Economy Impact of New Tires

[instarx]

There is a good technical article from Tire rack on rolling resistance and diameters of new tires and what to expect when installing new ones.

Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

From the article:

...the virtually instantaneous switch from worn tires to new tires (even if they are the same brand, type and size) will typically result in an increase in rolling resistance of about 20%. Since the automotive industry estimates a 10% increase in tire rolling resistance will result in a 1% to 2% decrease in vehicle fuel economy, drivers should expect to experience a potential 2% to 4% decrease in mpg.

Concerning changed tire diameter from worn tires to new:

...the vehicle's fuel economy (whether calculated by the driver after filling up or the trip computer) would instantaneously appear to drop by about 1.5% when fitted with new tires simply because the vehicle would actually have traveled 1.5% farther than it did when equipped with its recently removed worn-out tires.

[RPM]

Becoming an hypermiler made me a lot more sensitive to tyre changes. I can feel the car dragging a lot more on new tyres: it won't coast nearly as far and it won't hold top gear at the same low speeds as before. Last time it took me something like 5000 miles to get back to my previous numbers!

Even rotating used tyres front to back makes a noticeable difference for a couple of days while the tyres get scrubbed the right way.