Best fuel mileage 18's

The best 18” tire is a 15” one

Or move up to 19” I3 SLRR tires

Bridgestone Ecopia Ep500 155/70R19 Tire 84Q - Walmart.com

 

How many tanks have you used with your 18"? I'd do a minimum of 2 full tanks before panicking.

 

When did you change tires? Do remember that MPG falls in winter even without changing tires.

What actual tire size did you get? Any change in tire diameter will change speedometer and odometer readings, messing up your MPG calculations.

 

And - you might need to increase the pressure - PRIUS runs fairly high pressures, and if you're running the "recommendation" of the tyre place, they're probably a guess. Add a few more.

 

I doubt it that the choice of low rolling resistance tires is that great especially in the size you need.
I would contact tirerack.com and ask them, they are real good at getting back to you by email.

 

@djarmy2004 just curious: what are your current 18" tires?

 

I have a black 2016 and bought wheels that look like yours except they are 15 inch American Racing.

I haven't installed them yet.

 

I'm on 18's and currently getting anywhere from 55-57 mpg per tank fill up. Reseting trip A at every tank to verify. Using Nankang - Noble Sport.
I check my tire pressure often to make sure it good. Once it drops too much I instantly lose 5-7 mpg easy.

 

What pressure?

 

Tire pressure usually around 40/42 gets best milleage for me.

When it dips down close to 35 ish is when I see the mileage start to drop.

 

From what I can find the diameter of your tires are slightly larger than the stock tires and they are performance tires and not LRR.

It's a double whammy for gas mileage and winter. It can't get much better.

 

Is that by calculation - or a readout from the dashboard?

 

It's by trip calculations. After fueling I reset the trip meter.

 

Because it's calculated by the computer, from a lot of inputs across many many trips, it is only fairly approximate - most people's real calculations (litres/km or whatever) are between 6 and 12% optimistic - every brand of car I've had has been optimistic, funny that.

With the optimistic error of the trip-calculator, that probably means that 55-57 mpg would be lower.