Will going from 195 to 185 result in better fuel economy?

Good question, but you have to consider load rating. You would have to find 185 tire with load rating of stock 195 tire, and such a tire may not be LRR. I don't know though as I newer checked.

 

What are the revolutions/mile between the tires?

The reason I ask is on our older, 2003 Prius, I reduced the number of rev/mile by using over-diameter tires:

• improved steering - the larger diameter tires improved straight-line stability
• lower transmission losses - turning at slower speed, less stirring

I'm going to suggest going with just one pair of tires first and trying them on the drive wheels and later on the rear wheels. This will give you a chance to compare-and-contrast their performance.

In an ideal world, get two extra wheels for the new tires so you'll have six wheels total. You can then conduct the front and rear tests with the option of putting the old pair on do A-B-A testing.

Remember to measure a calibration factor during your testing. The indicated MPG, speed, and distance are likely to be off . . . significantly. Only by running a calibration test when any of the wheels/tires are changed do you have a chance to understand the effects.

When done, keep the best tire as a full-size spare and sell the other as "a full size spare." <grins>

Bob Wilson

 

Also keep in mind that the "65" refers to 65% of the width. So the 185/65R15 will be both narrower, and shorter (by about 13mm).

 

The narrowing is good, but the reduction in outside diameter is going to mean the engine rev's a little higher at any given speed. Also your speedometer and odometer will read higher. One (dubious) consolation: the odometer being high will mean the car's (and your) calculation of mpg will be higher.

That said, it's not a lot of difference.

 

I think this is a VERY BAD idea! what, am i the only one to think this? What could you possibly think you are going to gain vs. the cost of this (if it even would work)?

 

No! The stock has 89 load rating, the smaller tire is 86. No honest tire shop will put tires rated lower than stock.

 

looks like an accident waiting to happen.

 

For openers, your odometer and speedometer will be even further off than they are now, so any MPG calculations will be skewed. It'll look better than it is.

 

Minor correction:

The 'transmission' revs will be higher. The engine will still follow the operating line and provide as much power as needed to match the vehicle drag. A subtle difference but real due this being an HSD transmission. Everything else you've pointed out is spot on.

Bob Wilson

 

That is why you bring two wheels and two tires. <GRINS>

Bob Wilson

 

I wouldn't do it. I had thought about it but I think the smaller diameter and the extra load on the tires (more flexing) will hurt FE more than help it. It's not a safe way to go either.

 

My '06 came with 185/65-15, I put 195/65-15's on to correct the speedo, and get better mpg's(195's change the final gear ratio vs 185's and will give better mpg's, just don't start out abruptly)

Running two different sizes front and rear may set off the stability control gremlins, happened to me, was switching from 185's to 195's and the tech broke a tire, so they put the new ones on the rear (I wanted them on the front, they said they legally couldn't, and put them on the rear, I laughed and left) when I went over 32mph the ice quit, so I limped home and took only the wheels in for new tires, the shop was oblivious to the fact that they could have caused a fatal accident, but very concerned that the new rubber only go on the rear, (just a little off topic,)

 

Just thinking:

Toyota sells the 3rd gen Prius with standard 195/65R15 or optional 215/45R17. The latter's size is pretty close to what the 185/65R15 the OP's considering:



(These numbers are just theoretical, based on stated size. Actual numbers vary a bit.)

 

I'm actually thinking about going to 205/65/15s to reduce RPMs, increase handling, and hopefully increase MPGs. I've done this on several vehicles in the past with good results but this is my first prius and I'm still new to the technology. Car has 6K miles on it and last 2 tanks acheived 55 MPG with a mix of road conditions. I might be barking up the wrong tree.

 

In theory, a smaller width tire would be a more aerodynamic tire. This may be negated by Toyota designing around the tire they chose.

Passenger weight, plus cargo with the stock tires is about 850 pounds with the reduction in load with 185s, it will only be about 400 pounds. 185s make this a dual occupant car, with no cargo.

185s will alter the odometer and speedometer readings, how will you judge MPG if you can't trust the odometer or speedometer?

In theory a 185 70 15 tire would be nearly the same circumference as the 195 65 15, yet narrower, but sadly no one makes such a size.

 

First let's talk about the important metric, "revolutions per mile." This is not constant across any tire aspect and it is the only thing that counts. Now I am four years into this experiment:

What you see are three diameters of Sumitomo T4 tires:

• 919 rev/mi - the first set, recommended but undersized, they always indicated more speed and distance giving a false indication of good MPG.
• 886 rev/mi - the second pair, bought to replace two of the first set. The speed and distance were more accurate but on this model, slightly improved handling.
• 849 rev/mi - the third pair, now on the front for improved handling, the speed and distance show ~6% low. The indicated MPG is 6% low but the actual MPG is as good as what I've seen in the past. I use a GPS for speed and distance measurements or apply a 6% calibration constant.

Using a larger diameter tire means the transaxle rpm will be lower, reducing rolling drag from stirring losses. However, get one of the test tires to check for clearance and tolerance over speed bumps:



Bob Wilson

 

"generally" speaking, smaller wheels get better gas mileage due to lower torque demands to get the tire spinning. but remember traditional cars have a very narrow RPM band where it performs at best efficiency. lighter tires will do it and a somewhat higher revolutions per mile I am guessing wont be significant.

in the old days, economy cars came with 13' wheels due to poorer performing engines with low HP. this provided an acceptable level of acceleration while increasing performance.

keep in mind changing this will increase the inaccuracy of your speedometer