2017 Prius 4 with 17” wheels and added fog lights help/gas mileage went down

Different tyres probably.

Some reports show greater consumption by 17" wheels, whether it's weight or greater wind resistance. Plus the fog-lights could have changed the aerodynamics (and added a little weight).

You can see that those foglamps would be hopeless in actual fog - they're shining upwards in the photo, even glaring the camera.

It could also be that, with more of a sporty feel to the car that you're driving it a bit harder to enjoy the newfound sportiness.

 

Will swapping the 15 inch wheel for the 17 inch make the odometer read 13% lower for the same fuel consumption?

 

It will definitely make a difference, but it's impossible to quantify as so much depends on the routes weather and driving style to calculate.

 

If the 15" wheels had a 25" outer circumference and the 17" wheels have a 24" circumference, the added weight and the wider tires will definity effect mpg's, and if the speedometer is reading a 2-5 mph difference because of a smaller overall circumfernce, the mpg's are also going to be a lower number.

 

These generalizations may not all apply.

Rotating a heavier wheel/tire combo will consume HP, and therefore reduce MPG. Low Rolling Resistance tires are rarer in 17 inch sizes than 15 inch sizes. New tires will get reduced MPG for the first 5000 miles, even if they are the same make and model.

If you drive mostly above 50MPH, the fog lights will not reduce MPG much, but at lower speeds the engine will run more frequently to make electricity.

Somewhere in the depths of Priuschat, someone found the raw results of EPA testing for different trim levels one year, and the 17 inch wheels/tires were about 3 MPG less efficient.

 

Yes. Our 3rd gen 17" are real porkers compared to 15", something like 24 lbs vs 18 lbs. The fourth gen 17" are better in that regard, still fairly lightweight, but I'd suspect still a little heavier. Then the tires are heavier, and wider. Also, you have more weight further out, nearer the circumference.

And the weight is "unsprung". You can research that; it's too much for my brain cells.

All of these factors are at play to increase rolling intertia and resistance, resulting in maybe 3~5% drop in mpg. It's not terrible though.

 

What are the new tires? Did I miss that?

 

There are several things coming into play, first fog lights on/off wont make any difference the draw of fog lights off the electrical system wont be noticeable outside of a testing environment. Next atmospherics (air temp, humidity, density and pressure altitude) can play a huge factor in fuel economy so if one week its 90 and humid and the next is 68 and low humidity will make a huge difference. So even if you run a month long "test" in April and another in July its not an apples to apples unless you are correcting to a standard (59 degrees and 29.92 inHg) as a engine continually varies fuel mixture based on many things. Next are you doing the MPG calculations your self or just letting the cars systems do it? I often get 1-3 mpg variance per fill up vs what the car says and at times even a little more but not often. And finally there are a million different variables on wheel and tire combos, just because you up on size doesnt mean the weight goes up, many 17" wheels are much lighter than 15" steel wheels and the actual circumference of both will be within 1-3% of each other, if you go to tirerack's web sight (cant post web address yet) they have several good tech articles on upsizing wheel/tire combos.

 

same question here. a tire with a 200 tread wear will drag the fuel consummation down vs a 600 with low rolling friction as on my 4 gen 3t