The OEM tires have a 65 aspect ratio, not 60. Assuming that your new tires are 195/65/15 Integritys, the revs/mile is 836. That is about a 2% difference. In my experience, the odometer over registers by 2% with the OEM tires so your new tires should make your speedo/odo more accurate. Your 5 mpg drop is about 10%. The other 8% is most likely due to the new tires. There is a huge hit associated with new tires. Give it 1000 miles or so and see if your mileage doesn't increase. It probably won't all come back due to the wider tread width which means more rolling resistance.

Dan

 

Now to account for the other 8%, what is your air pressure in the new tires. You should inflate them to just below the maximum rated pressure (see the rating on the sidewall) but 2 lbls less in the rear.

If they are inflated to the pressure set by Toyota, your MPG will suffer. I run 42/40 in my Michelin MXV4+ tires. It causes the ride to be a little harder, but improves handling by making the Prius less vulnerable to swaying in the wind as well as improving the MPG.

JeffD

 

In our town, at the oldest quick-lube place, the owner works down in "the pit," doing all of the actual oil changes himself.