Touring Edition Tire Upgrade

[Boo]

Does anyone know if the RPM of the 205/55/16 size is different than the RPM of the OEM 195/55/16 size? (I guess the question is whether the tread width number is relative or absolute). EDIT: The middle number is a ratio, so if the ratio remains the same but the tread width changes, then the RPMs should change too, right?

I'm wondering if the RPMs are different --- what the effect is on speedometer, odometer and mpg readings.

Sorry for the question. I just never "got it" when reading the posts on this tire sizing/RPM issue in the other tire threads.

EDIT: I think I answered my question via google. See subsequent post below.

[wblovett]

<div class='quotetop'>QUOTE(Boo @ Oct 21 2007, 10:59 AM) [snapback]528454[/snapback]</div>

Does anyone know if the RPM of the 205/55/16 size is different than the RPM of the OEM 195/55/16 size? (I guess the question is whether the tread width number is relative or absolute).

I'm wondering if the RPMs are different --- what the effect is on speedometer, odometer and mpg readings.

Sorry for the question. I just never "got it" when reading the posts on this tire sizing/RPM issue in the other tire threads.
[/b]

Sorry for the slow response. Been away from the forum for a while. The 205/55/16 does not affect the height of the car, the speedometer's accuracy, or mileage (so far as I can tell). The greater width is not obvious, but makes the car appear more proportional, in my mind. I'm running them at 37 psi (max psi of 40 with this tire), although the tire place installed them at 34 psi.

[Boo]

<div class='quotetop'>QUOTE(Bill_Lovett @ Oct 21 2007, 08:58 PM) [snapback]528597[/snapback]</div>

Sorry for the slow response. Been away from the forum for a while. The 205/55/16 does not affect the height of the car, the speedometer's accuracy, or mileage (so far as I can tell). The greater width is not obvious, but makes the car appear more proportional, in my mind. I'm running them at 37 psi (max psi of 40 with this tire), although the tire place installed them at 34 psi.
[/b]

Hi Bill,

No problem. Thanks for the response.

I'm not sure I understand it all, but I think the height of the tire IS affected, albeit very slightly (imperceptible to the eye). I got curious about this question a few minutes ago while watching the 7th game of the Red Sox / Indians series, so I googled it. I found a tire RPM (revolutions per mile) calculator online and here are the short results:

The OEM 195/55/16 tire has 825.04 RPMs. The 205/55/16 tire has 810.68 RPMs.

This means that the 205/55/16 has a greater circumference of 1.77%. Not much.

So, assuming the Prius Touring odometer is accurately calibrated to the OEM 195/55/16 tire size, this should mean that with the 205/55/16 tires on your car:

[EDIT 10/23/07: Based on prim2's comment below, I realized that I was misreading the terminology in the online tire size/rpm calculator that I had used. So the following statements are wrong, i.e., the sequence of the two figures given in each statement should be REVERSED.]

* If your speedometer shows you going 60 mph, then you are actually going 58.96 mph. [EDIT: should read speedometer 58.96 mph, then actual 60 mph]

* If your odometer shows you went 100 miles, then you actually went 98.23 miles. [EDIT: should read odometer 98.23 miles, then actual 100 miles]

* If your MFD shows you averaged 50.0 mpg, then you actually averaged 49.1 mpg. [EDIT: should read MFD 49.1 mpg, then actual 50 mpg]

Anyone should feel free to tell me if any of my calculations or my express or implied assumptions are wrong.

One way that you could test all this in real life is if there's a route that you always take (e.g., to work), then just jot down the measured miles of the route with your OEM 195s on the car, and compare that to the measured miles of the route with the replacement 205s on the car. This comparison should show the 1.77% difference (assuming all other things being equal, like tire inflation psi).

[topkick]

<div class='quotetop'>QUOTE(Bill_Lovett @ Sep 14 2007, 05:20 PM) [snapback]512745[/snapback]</div>

Have been unhappy with hard ride of my 07 Prius Touring Edition since driving off the lot, and finally decided to change out the Bridgestone tires that came on it. Unfortunately, most of the tire threads on Priuschat address the non-Touring Edition Prius. Still, from what I read, I figured I would be safe going up to a 205-width tire on the existing 16-inch rims. Had Pirelli P4 Four Seasons in 205/55R16 installed today and the difference in ride is astounding. Quieter, softer, better handling. I'm very happy with them. And, I am so glad that we have a site like this one to turn to for advice. Thanks guys.
[/b]

Hey Bill,

I have another question for you (or anyone that knows). Does the Pirelli P4 tire come from an outlet like Sam's club or one of the others? I see where they can be bought online at several places but then have to get them shipped and locally mounted. Thanks for any info.

[prim2]

[quote=Boo @ Oct 21 2007, 08:36 PM) [snapback]528619[/snapback]</div>

The OEM 195/55/16 tire has 825.04 RPMs. The 205/55/16 tire has 810.68 RPMs.

This means that the 205/55/16 has a greater circumference of 1.77%. Not much.

So, assuming the Prius Touring odometer is accurately calibrated to the OEM 195/55/16 tire size, this should mean that with the 205/55/16 tires on your car:

* If your speedometer shows you going 60 mph, then you are actually going 58.96 mph.

* If your odometer shows you went 100 miles, then you actually went 98.23 miles.

* If your MFD shows you averaged 50.0 mpg, then you actually averaged 49.1 mpg.

Anyone should feel free to tell me if any of my calculations or my express or implied assumptions are wrong.

[/b]

I haven't had my coffee yet, but wouldn't he be going faster than indicated with a larger circumference? Since the speedometer is just a wheel tachometer and assumes travelling one OEM tire circumference in distance per revolution, if the circumference is greater than OEM, speed is greater than indicated. I figure 61.06 actual]60 indicated.

[Boo]

[quote=prim2 @ Oct 22 2007, 10:04 AM) [snapback]528793[/snapback]</div>

I haven't had my coffee yet, but wouldn't he be going faster than indicated with a larger circumference? Since the speedometer is just a wheel tachometer and assumes travelling one OEM tire circumference in distance per revolution, if the circumference is greater than OEM, speed is greater than indicated. I figure 61.06 actual]60 indicated.
[/b]

Hi prim2,

Hmmm. Even with my morning coffee I'm having a hard time figuring this out. What you say makes sense to me. I'm going to revisit this later today when I return home and, hopefully, am completely sober. In the meanwhile, here's the calculator I used --- maybe I'm misreading it (wouldn't surprise me) or maybe the calculator is wrong (that would suprise me):

http://www.csgnetwork.com/tireinfo4calc.html

At the top portion of the calculator, you enter the OEM and new tire sizes and the "foundation" base speed (e.g., 60), then press "compare".

[Boo]

[quote=prim2 @ Oct 22 2007, 10:04 AM) [snapback]528793[/snapback]</div>

I haven't had my coffee yet, but wouldn't he be going faster than indicated with a larger circumference? Since the speedometer is just a wheel tachometer and assumes travelling one OEM tire circumference in distance per revolution, if the circumference is greater than OEM, speed is greater than indicated. I figure 61.06 actual]60 indicated.
[/b]

Hi prim2,

I think you're right! Thanks for pointing out my error. I had misinterpreted the terminology used in the online tire size/rpm calculator that I had used. I have since edited/corrected the statements in my original post (see above). Let me know if you think any of my edited/corrected statements are wrong.

[prim2]

All looks great. Very handy link, by the way!

Cheers,

Geoff

[ceric]

Guys:

Discount Tires is having $100 off for a set of 4 tires. Very good deal!
I just got a set of 4 Sumitomo 205/60R15 to replace my poor Goodyear Integrity 185/65R15
(worn out at 22K - I don't inflat them to 40psi) for ONLY $192 ($0 shipping, $0 salse tax). (both are at 24.5" diameter)
You can easily find a local shop to mount them for $15/per + $4 stem/disposal
See here.
http://www.discounttire.com/dtcs/findTireSection.do
I was told that you could have the same deal at your local America's Tire. Not sure about this personally.

Tirerack.com is a very good resource for researching tires. They normally have better price than Discount Tires.
However, with $100 off, Discount Tires has better deal NOW.

[Dahgu]

Boy. I must still be in SRT8 mode. I spent $700 on a set of tires for my new Prius and thought it was a steal.