Doing a small survey and need to get a couple of answers regarding this.

Do you keep your tires inflated regularly?
Whether yes or no, have you noticed a difference in gas mileage when they are inflated versus deflated?

Yes, you do notice a difference; I go to the full tire pressure when I am on all pavement usually in the summer and then air down a bit if I am going thru mixed terrain. Differing on the vehicle, you can gain 1 MPG more.

indeed... the bag of dick that the dealership had changing my oil had my tires inflated to 40psi+... hence I was clocking 44-45mpg all the time. Had someone with experience change the oil in the car (they do a total service) and lowered the pressure back to the recommended 34-35 and my mileage dropped to 38-42mpg. And the car handles better.

I think mpg will vary more due to driving style but no doubt an under inflated tire will decrease fuel mileage was well.