Replaced tires - MPG Plummeted - is this normal?

Lack of tire wear is proof that pressure up to the maximum not a problem. I ran 44/42 for 5 years with my 2004 Prius. It was no big deal. Wear was even across the tread.

Automaker recommendations are based on a balance of comfort, performance, and price with the factory tires in mind.

Tiremakers know their tires and design them to retain their proper shape up to the maximum listed on the tire itself. In other words, you are not "over inflating" unless you exceed that.
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The type of tire can make a big difference. I don't know the specifics of either of your tires, but switching from a low rolling resistance tire to a soft performance tire will cause a big hit.

Simply switching to new tires of the same type will cause a small drop in mileage while they break in.

Tom

 

1. Unless the tire dealer has a [x]-day satisfaction guarantee, I wouldn't spend even more money just because fuel mileage has taken a hit.
2. Yes, although you might not regain completely your previous fuel economy. As John suggests, the rolling resistance of your new tires may be higher than that of the old ones.
3. No worries about inflating them to 51. There are many that inflate Prius tires to max sidewall (and even beyond). I have seen absolutely no report of any with excessive wear in the middle of the tread -- the pattern that might be caused by overinflation. On the other hand, there are many reports of outer edge wear when owners have kept pressures in the 30s. The only downside to inflating to max sidewall pressure is a stiffer ride. You should see a substantial gain in fuel economy by pumping them up.

 

There's no one source of rolling resistance info for all tires, and there may not be any source for the particular tire you're interested in.

You can google "low rolling resistance tires" and come up with a dated list of some low rolling resistance tires.

But I'd rather go with apples to apples tire tests, the two most recent and best of which are TireRack.com's August 2009 test of several, mostly low rolling resistance tires, and Consumer Reports' November 2009 comprehensive tests of a wide variety of tires.

Here's a link to TireRack.com's tests of low rolling resistance tires:

When Round and Black Becomes Lean and Green

And here's a scan of Consumer Reports' November 2009 tire test Recommendations and Ratings:

Consumer_Reports_Nov._2009_Tire_Tests-1.doc

Read the tire tests, consider all the factors (handling, braking, wet performance, snow performance, rolling resistance, etc.) that are important to you and choose your tire.

Note that many of the tires tested do not come in the OEM 185/65/15 size, but are probably available in the next closest size of 195/60/15.

Personally, when I get new tires I'll probably be going with the Hankook Optimo H727 in the 195/60/15 size because it's the best performing 4 season tire (e.g., its performance on snow and ice is equal to or better than most of the Winter tires tested by Consumer Reports) and it's relatively inexpensive (about $72 to $84 a tire depending on where you get them from). But note that its rolling resistance is probably higher than most of the tires tested in that TireRack.com tire test.

Good luck.

 

What Tom said.

 

My MPG plummeted 20% for 10k miles when I switched to Falken Zi-Ex, but only dropped 3% for a couple thousand miles with my next switch to Michelin Hydroedges.

I'm not trying to say the Falken's were bad, just that tyre choice makes a big difference. Tyres that in general that are soft, meaning great handling and short life, will often be low MPG. That is my rule of thumb anyway.

 

You forgot to mention reduced handling. Less tire gripping the road from higher psi = longer time to stop the car, and not as grippy when turning, so don't drive as fast as you did before thinking you'll be able to stop or turn in time, could be dangerous. I've read that the speed that could cause hydroplaning when going over water is increased though, which means you can go faster than before, without the risk of hydroplaning, even though, don't go speeding around, drive at safe speeds.

 

I'm wondering if wear on tire makes the car think it's getting better mpg. Are you basing your mpg on the screen's readout, or are you actually calculating by dividing the miles traveled by how much gas you put in the tank? Also smaller tires from wear and tear = higher rpm to go the same speed, the prius could be thinking it's going 70mph when it's actually going 68, which would trick it into thinking it's traveling more miles on the same amount of gas, though I guess if you're using your odometer to count how many miles you're going, you'd still get less miles traveled because of the larger new tires. Maybe try using a gps or some other method of calculating how many miles traveled and compare it to the miles traveled of the odometer.

 

Rolling resistance decreases with reduced tread because a large majority of tire rolling resistance (60%+) comes from the tread. Some tires incorporate sidewall technology to reduce hysteresis while others may only change tread compounds.

The GenII Prius speedometer reads 1.5mph-2mph too fast from the factory. Worn tires or those with more revolutions per mile than the OE Integrity tire will increase this error as will very worn tires.