MPG decrease from 49.6 to 33-- Yokohoma (stock) to Assurance FuelMax?

1) Check the tire pressures on your new tires. You could be running at low pressures. Tire installers often suck and set the pressure on the tires to the door jamb irrespective of current temperature, resulting in low pressure when you drive it. I forgot to check pressures when my new FuelMax's were put on and found them at about 30psi. My fuel economy improved noticeably once I pumped them up to where I wanted them.
2) Maybe they screwed up in another way and your tires aren't properly balanced or something.
3) You've moved so your driving conditions will have changed. If you're driving shorter trips in heavy traffic with the A/C on you'll be much less efficient.
4) It could merely be a coincidence with the tires and now your 12V battery is shot or your new environment has exposed a problem. (Or maybe a neighbor is siphoning your fuel.)
5) To be picky, "down 17mpg" is meaningless since the "value" of difference between mpg numbers depends on the starting point. You're up 0.01 gallons per mile.

PS I presume you meant Goodyear Assurance FuelMax

 

I never had the yokohama tires but stock Assurance FuelMax and am having no issues with MPGs. maybe if I get the yokos I can gain 17 mpg lol, i would buy today if that was the case

 

You don't say where in CA you moved to and how your commute or local driving has changed from where you used to live. That could be a big factor in your mpg's.

 

If you don't know the previous tire pressures or the current tire pressures you don't know that only the tires have changed. That would just be an assumption. If they've had to inflate one of the tires you could have a slow puncture or slow leak in the valve. Please check your tire pressures. If they have to be raised get a new measure of fuel economy.

(Also, are the tire sizes the same? Larger tires have higher rolling resistance.)

But, even if you went from high pressure on old smaller tires to low pressure on new larger tires it's still an excessive drop.
Anyway, please post answers here to the PriusChat mileage questionnaire (Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat) so people with more knowledge can help you.
It might be a combination of things, including coincidental failure of your 12V battery.

 

I'd guess something went horribly wrong with your tire change (no duh).

1) Check your tire pressures. If you need to go out an get a pressure gauge, do so (they are useful to have).

2) Regardless of what the dealer or other place may have done, set your tire pressures to what YOU want. Many of us, who like a combination of longer tire life and better MPG's fill the tires to Maximum cold PSI or close to Maximum cold PSI. I don't notice a difference in ride quality, but my Yokohamas lasted 50,000 miles and still had a little life in them when I traded tires.

3) If, after checking and adding air to your tires, your MPG's are down, I would bring the problem to

A) Shop A that put your tires on, to give them a chance to spot their mistake and make it right....OR
B) Take it to Shop B and have them look at it. They will be much more willing to notice and point out Shop A's error since it will likely mean that you will frequent Shop B for your future maintenance or tire needs.

It could be that when the tires went on, something else went wacky. A bad alignment, way underinflated tires, something went awry and you have brakes that are sticking, etc.

I'd address what I could fix (1 and 2) and go to step 3 as soon as I could eliminate 1 and 2 as the cause.

 

I suspect something happened to your Prius during your tire change, and that it's not the tires causing the lower mpg's. Could it be brakes, alignment, transmission or combo of things? Even non-LRR tires wouldn't cause this much of a drop.

 

I agree. Taking care of the tire pressures will prevent them from saying "oh, the pressures were low, we filled them and it should be good to go". This eliminate one extra trip to the place that installed the tires, because

1) the tire inflation will fix the problem (unlikely).
2) It will prevent them from saying the pressure IS THE PROBLEM, when it is something else.

 

I'm wondering how many miles were in the trip computer for it to display 49 mpg?

Perhaps the previous owner had less of a lead foot? If the trip stored the last 3,000 miles at 49 mpg, then 6 weeks of you hammering around won't have reduced that much. Put new tyres on, reset trip and carry on driving as you have previously and all of a sudden your true mpg's are displayed.

 

There are TWO versions of the Goodyear Assurance Fuel Max in the stock size for the Prius. The OEM one weighs 19lbs and has a pressure rating of 44psi. The other one weighs 18lbs and has a pressure rating of 51psi. The size is identical, and both are "LRR". You can see the difference on Tire Rack.

 

One is S rated and the other is H rated. The 185/65/15 only comes in an H rating I believe. That is kinda weird though since the 185/65/15 does not need such a high speed rating. lol