Prius MPG off by 7%
 

I have a 2005 Prius. We were getting over 50mpg. I bought new tire (37,500) and the millage dropped to maybe 48 but started to come back up as the tires wore in. Just recently (last couple of months) the millage is around 47. No matter how hard I try I can't get back to 50. Also within the last couple of months I have noticed that all of the gas pumps in town say "contains less than 10% ethanol". I know ethanol has less energy per gallon than gasoline, but do you think the ethanol could be hurting my millage that much? Is it possible that maybe the battery is not working as well with age and millage (75,000)?

Thanks for the input ... Michael

I think that battery is probably fine, but it could just be one of those things. Mileage is never consistent with weather, random stuff, all of that, but the ethanol might be making a difference too.

I'm assuming you've checked all the normal stuff?
Tire pressure.
Dragging brakes
correct oil
Correct transmission oil + level
missing underside aero parts

Also; is 75,000 miles getting close to plugs?

Hello Michael,

The new tires could have worse rolling resistance (make sure to check the air pressure), and/or they may be heavier. Heavier tires has a "double" penalty, since you have to spin them and they are part of the overall weight of the vehicle.

The ethanol could also be part of the mix -- I don't suppose that there is a winter blend down there, but for sure up here in New England, the winter blend of gas gets noticeably worse mileage.

Check out the driving tips!

I thought tires first also; then I realized he has 37,500 miles on the "new tires" and his FE has increased since he installed them.

Mike, you hit the nail right on the head. Florida started phasing in ethanol mixed gas in February. Your mileage loss is right on target for the lower energy contained in the new mix.