185s rather than 195s for snows...?

Take a look at the Michelin X-Ice Xi3. Look at the overall diameter for the 195/65/15 and the 205/65/15.

Click on the "specs" tab
Michelin X-Ice Xi3

 

Because a wider tire is better for rolling resistance unless traveling at high speeds where aerodynamic drag becomes more important than rolling resistance. It sounds counter intuitive but the math make sense. The GenII came with tires that are essentially too small for such a heavy car (185/65/15). This means the tire is likely to deform more as it tries to support the weight of the car. This increases deformation which turns into heat. All of the energy sued to deform the tire and create heat came from gasoline! A wider tire tends to deform less thus more energy is used to move the car instead of deforming the tire. The GenIII comes with a 195/65/15 as does the PIP. This is a larger diameter tire and it has a higher load rating. The higher load rating means it can support more weight. This tire will deform less than the 185/65/15 on the same car. A bonus is the car handles and stops a lot better as well.

Going up to 16" and 17" tires decreases fuel efficiency for different reasons. I'm still trying to work out those details but its mostly to due with weight and tire construction.

 

Excellent. Half an inch. That should be a significant help. Thanks.

205 is narrower (tenth of an inch) than the 195 so might offset the mileage hit of the snow tires.

 

but doesn't a narrower tire have less contact with the surface and thus LESS rolling resistance? isn't that the reason for inflating tires above what Toyota recommends? what about the Gen I? that had pretty much the narrowest tires i've seen on a car in the last ten years.

 

No. The contact patched reduction is minimal. The real increase in fuel economy associated with higher air pressure is based on the reduction in tire deformation. With higher air pressure the tire becomes stiffer and more resistant to bending. When you put too much air pressure into a tire you can cause it to skip over rough road surfaces because the tire has a more difficult time bending/conforming to irregularities in the road. This is one of the reasons not to run very high pressure when driving on rough roads. Not only is it uncomfortable but it can reduce fuel economy because every time the tire skips you lose forward moments.

More reading:

Check out page 11.
http://www.edccorp.com/library/TechRefPdfs/EDC-1038.pdf

What is hysteresis?
Rolling resistance - Wikipedia, the free encyclopedia

What does Barry say.
Barry's Tire Tech

It works for bicycles racing as well.
Technical FAQ with Lennard Zinn: Tire pressure

 

hmm, good to know!
though this thread was originally about snow driving, so you have to think about how the narrower tire is better for traction in snow as well…

 

I agree but the difference in width is pretty small and is outweighed by the reduction in traction in all other conditions as well as reduced handling.

 

a ok that makes sence

now i know for me this is not a problem because i dont care about handling and my air compressor can not handle the high tire pressure i am on so no deformation here :

so 175 10000PSI wil be best i geuss

 

We know the Prius speedometer overstates mph by about 2 mph. Going from a 195/65/15 to a 205/65/15 should correct that according to the Tire Size Calculator.

 

Shouldn't that be the other way around? I looked at the tire calculator and it shows the 185's makes the speedo show slower (smaller circumference) which is what we need since I think my car speedo is around 1.5 to 1.75 mph "generous" from using my aftermarket GPS. The 185's should make the car's speedo perfect. The mileage will show worse but at least it'll be accurate.

I have a set of 185 snow tires left over from my Gen ll. According to F8L they'll bolt on (thanks) so I'm going to use them. Sadly I'll also be gimacing for having to use the heavy steel wheels that they have.

 

If you go to a tire with a larger OD, it will do fewer rotations over any distance, and accordingly the car will think you're going slower, and the speedo will reflect this. So ProximalSuns is right on this.

Still, I'd be very inclined to stick with the stock size, and that speedo error. It's no big deal, the odo should be accurate. The object with the speedo fudging is to reduce speeding, not a bad idea.

That said: in my situation the stock tire is 215/45R17, but I got snows in 195/65R15. I'm aware the latter has slightly larger OD, and adjust speed downward slightly when they're on.

 

With the larger diameter (25.5" for 205 vs 25" for 195) on the snow tire, I'm gaining a half inch of ground clearance for driving in snow and slush. That is my reason for going with the bigger tire. The correction to the speedometer is a bonus.

As far as purposely making the speedometer inaccurate, there's no excuse for that. Especially bad practice on the Prius where more drivers likely go the speed limit to use less gas and not bug other drivers by going under the speed limit. Instead Toyota has them driving under the speed limit, creating traffic problems and giving Prius drivers bad information and bad reputations for...driving under the speed limit and holding up traffic.

I wonder what the diameter size change is going to do the my actual mileage and the Prius reported mileage? Will both go up?

 

Most vehicles tend to show a 1-2mph speedo error.

The actual ground clearance difference between a 25.5" tire and a 25" tires should only be about .25" because the extra .50" difference in diameter has to be cut in half because part of the extra tire diameter is at the "top" of the tire and the other is at the "bottom" of the tire. Regardless, the extra height is beneficial in snowy conditions.

 

5.3" to 5.55" ground clearance. Small but every bit will count on getting a FWD car through the piled up snow on the parking lots and the piles in the center of the lanes.

 

I believe that inaccuracy is legislated.

 

No it's not. Speedometers must be accurate +/- 10% at 50 mph (for US) but there is no legislation that requires it to be inaccurate which is what Toyota seems to be doing.

Since it is all electronic, it should be easily correctable and should be corrected.

 

Not sure. There appears to be a worldwide concensus that speedometers must never indicate a speed lower than actual:

Here's the full wiki article:

Speedometer - Wikipedia, the free encyclopedia

There's mention of US commercial vehicles having to be within 5%, but I think that is only on the overestimating side, ie: the above stipulations come first.