35 to 40mpg

The tires are the reason why. some of it is that you do spend a little bit more energy trying to change the moment of inertia of those bigger tires. you also have a little bit more rolling resistance because they're wider. But you also have a false fuel economy and speedometer reading now because those wider tires with the same profile is actually a higher profile than the stock tires. 65% of 205 is more than 65% of 195. Your tires are taller. you don't need to change the tires. Use them till they're worn out but don't buy oversized tires for this car again. I actually prefer a 55 profile. I have 50 profiles on four priuss right now. We have to be careful about crashing the front skirt. We need to go across bumps one tire at a time instead of head on. a 50 profile gives us the best handling but the speedometer is off by 10% and the fuel economy is falsely inflated. I don't really care about the speedometer and feel the economy too much. Just trust me I'm not going bigger with tires for any economy car. Besides these rims are 7-in wide all you're doing is pinching the bead and not getting the full use of a wider tire anyway

 

Let me venture a guess here: It isn't the car. It isn't the tires. It isn't the battery. It's the use case.

To illustrate, here's my fuel log, which catalogs every fill and every mile driven since I bought this car new in February 2014:

The red bar there is a fillup on Feb 10, 2018 at 37.61mpg over half a tank (246.9 miles), my worst fill ever.

Just over two months later on April 12, the car returned 61.89mpg over 571.7 miles. What happened? Was something wrong with it in February that got corrected by April? No. The difference is: the April tank was a long highway drive in mild to warm weather headed east with favorable winds. The February tank was lots of short trips between 2-3 miles in length in freezing temperatures. The car isn't magic; fuel economy depends to a great extent on the external conditions that we impose by choice or by circumstances beyond our control.

So, my advice is: wait a while. Go on a long highway drive at moderate speed and see what fuel economy the display reports, and then fill up and calculate actual fuel economy. Correct miles driven for your larger tire diameter. Start keeping track of your fillups and miles driven so you have something to which you can compare current tank averages; otherwise you're shooting in the dark. Be honest with yourself about what has changed in your use case--whether that's average daily temperatures, increased number of short trips, whatever. It's easy to "see" a problem where none actually exists because our data are faulty or non-existent.

 

It'll knock down your mpg for several reasons:

1. New tires, in the same (specified) size, can lower mpg by 5~10% all by themselves. I went through that, going from well worn, 10 year old Michelin Pilot HX MXM4 215/45R17, to brand-new Michelin Primacy MXM4 (in same size). It's taken about a year to get out of that hole.

2. Your new tires have larger outside diameter. You're effectively fooling the car, making it think you've travelled less distance.

3. Again with the larger outside diameter: you're changing the car's "gearing", not sure what that's doing to it's efficiency.

4. The make/model of the new tires can be a BIG factor in the fuel efficiency. There's a wide range of "rolling resistance" in tires. What were the old tires, and the new one?

 

Hmmm ... that sounds like some kind of severe aerodynamic problem. Is everything in place under the car? Stock wheels and wheel covers?

 

Brakes be draggin

 

OP, let me just take a guess in a wildly contested guessing game. Since you mentioned in January you did an oil change, new spark plugs and new tires... Are you a victim of knockoff spark plugs from eBay or Amazon?