17 inch tire buying

<div class='quotetop'>QUOTE(B Rad @ Jul 17 2006, 12:53 PM) [snapback]287695[/snapback]</div>

That's easy - all other things being equal, the narrower tire will have less rolling resistance, and that's what REALLY matters when it comes to fuel consumption. In fact, on this car, no way I would use a 215mm cross-section tire if you care at all about fuel use.

Wider sizes are about handling, not about fuel economy. The weight difference you're talking about is trivial, and in fact, irrelevant - the bigger the contact patch, though, the greater the drag, and that DOES matter.

Within the same size, you need to focus on rolling resistance, which varies widely from one brand / tread design to another; but between sizes, the smaller cross-section is almost always the winner, unless you want to go around corners faster.

 

The wider tires, if all other factors are equal, will handle better and provide more traction. They will also have more "road feel" (some people like that and some people do not like it at all). Unsprung weight takes more energy/power to turn.

In reality, the actual differences between two pounds and minimally more rolling resistance is so slight that the difference will not be able to be accurately quantified.

I would proceed based on the following:

I would initially favor the lighter tire as the stock suspension should work better with a lighter tire, shocks and springs should also last minimally longer.

However the actualy determining factor for me would be which tire footprint more closely matches the exact width of the rim? In a perfect world, a seben inch wide rim should have a tire on it that has a footprint of seven inches. The sidewall width does not matter. The specified width and the measured width are often two different things on tires, however rim width is usually exact.

As an example....

I run 28 x 12.5 x 15 inch tires on the rear of my Camaro. The rims are 15 x 10.
The actual tire size is a 28.1 inch diameter, 12.6 inch sidewall, and 10 in tread (footprint). The rims are exactly 15 x 10. The tires weigh 24 pounds each.

My front tires are 28 x 7.5 x 15, and actually measure 28.0, with a sidewall of 6.9, and a footprint of 5.5, with a weight of 20 pounds per tire.

Tread pattern and rubber compound will likely have more impact on measurable rolling resistance than the minimal difference in size.

 

Will you have to put larger car springs (taller) on to fit the larger size of these new wheels?

 

The only thing that matters in regards to height is the size of the tire. If the actual tire diameter is within .1 (unless the Prius is more fussy than anything else on the road), then the on boad computer should function fine and the speedo should be reasonably accurate.

Whether or not the wheel is taller or shorter should make no difference as long as the tire is the same OD. taller wheel may actually provide more clearance (cooling) for the stock brakes.

 

<div class='quotetop'>QUOTE(tnthub @ Jul 18 2006, 07:14 PM) [snapback]288435[/snapback]</div>

Ok so how do you fit the same size tire on the larger rim?

 

<div class='quotetop'>QUOTE(jrct9454 @ Jul 18 2006, 05:49 AM) [snapback]288071[/snapback]</div>

Actually... the force required to move the extra rotational mass per wheel is the mass times the radius squared.

I=m*r^2

Adding weight to the outside of a rotating object (eg. heavier tires) has a far greater effect than putting the same amount of weight inside the car.

I agree that 2 lbs difference at approx 1 foot radius is not much compared to the almost 300ft-lbs of torque that the Prius has on tap, but by the same token neither is 1 cm of tread width.

The thing is, it's all cumulative.

 

<div class='quotetop'>QUOTE(Cheap! @ Jul 18 2006, 10:17 PM) [snapback]288493[/snapback]</div>

OD and ID are two different measurements....

My camaro came stock with 16 inch rims and 26" tires. My first major change was to 15" rims and 26" tires, so the sidewall height on the new rims was taller than the stock sidewall height. This is generally bad for cornering, good for ride quality, and good for accelerating quickly from a standing start.

However at this point, for me, all the variables have changed, but in a normal car like the Prius, larer rims and a shorter sidewall will provide better cornering with probably more road feel and a little more noise and harshness. Harder rubber compounds which usually offer less rolling resistance will add to the effects. Softer rubber compounds will lessen the effects. The construction of the actual sidewall will also have a large effect on ride quality and cornering. Tread patterns will have a big effect on snow, rain, andry pavement.

In my opinion, it is still a good thing to have a set of bonafide studded snow tires from Novemeber to April where I live in Maine. The Camaro I do not drive on wet roads (too much power) so the only thing I care about is dry weather traction. my Sebring I use all season radials on with shorter sidewalls because it is fun to drive. I also have a Bonneville which i use a taller sidewall on as the suspencion is a bit tight. My geo Metro has tiny 165 14" tires on it...

 

If you put 17" tires on a car that normally comes with 15" tires, don't you have to somehow modify the odometer and computer to show the right mileage?

One revolution of a 17" tire will cover 2*pi*17"= ~107 inches, while a 15" tire will go ~94 inches.

Simpler still, the odometer and speedometer, if not somehow recalibrated, will show you only 15/17 of your real distance and speed. Right?

 

If you go to Tire Rack and click on the spec for the tires you want you can find the revolutions per mile (this may be a bit more accurate than diameter because it's a dynamic measurement). If you keep it within say 1% of the stock 855 you should be fine. If you are looking at tires that Tire Rack doesn't sell, Toyo for example, you can usually get the information from the tire manufacturers web site.

I recently changed tires and went to a size that was 2 cm wider (205 vs 185), weighed 2.8 lbs more per tire, and was 847 vs 855 revs/mile. I lost about 2 or 3 MPG at lower speeds. No way to know how much of that is due to extra width and how much is due to extra weight. At freeway speed the mileage seems almost the same, I think because aerodynamic drag overrides other considerations.

 

<div class='quotetop'>QUOTE(tumbleweed @ Jul 21 2006, 02:39 PM) [snapback]290049[/snapback]</div>

Couldn't some of that "loss" be attributed to extra friction from the tires being "stiickier" than your previous set? Also, if I remember correctly, tires have a break-in period?

 

<div class='quotetop'>QUOTE(Ken S @ Jul 23 2006, 06:50 AM) [snapback]290887[/snapback]</div>

Certainly. Sometires have stickier compounds, some have more surface area, some are heavier (more rotating mass), some tread patterns are more efficient than others... All of these are variables in traction and miles per gallon. The break in period for tires is extremely short. For street tires they simply need to warm up enough once and go a few miles. That is it.

For my Camaro with racing tires... It takes a couple of good burnouts to get the oils hot and the tires seated correctly. I have done it with one really big burnout but it looks pretty stupid immitating John Force in a street legal vehicle.

 

Thanks to all, I'm learning much about rims, and tires.

Wheels = rims
Small wheel + big tire = x
Big Wheel + small tire can also = x

Got it.

Any advantage to a shorter tire wall? Should I go to a 20" rim with short wall tires to get better FE, or should I go to a 12" wheel with tall side walls to get better FE, or should I just stay with stock? Anyway I cut it I do want narrower tires. Now they are stock 185 but I would like to go to 155 or even 145. If I could just find out if they would fit on my rims or do I need different rims to handle the 145/155 width?

 

A shorter sidewall will usually rider harsher than a taller sidewall. It will also roll less when cornering generally providing more stability in turns. However the stability of a lower sidewall may be offset by a more narrow tire and at a certain point (based on vehicle weight, tire and wheel height, and use), a more narrow tire will become unstable to drive on in real life scenarios...

The front tires on my Camaro are very narrow. I need ro adjust my driving habits accordingly. The Prius is not a lightweight vehicle. If it were me, I would put a slightly more narrow tire on the rear wheels as all they do are roll. The fronts I would just match precisely with the rims as it appears this matching may have some affect on tire wear.

 

<div class='quotetop'>QUOTE(tnthub @ Jul 27 2006, 07:44 AM) [snapback]293089[/snapback]</div>

That would work, but do I need new rims to accommodate a 165, 155 or even a 145 wide tire? I know that on the standard rim that comes with a 185 wide tire you can go wider to possibly a 205 wide but when going smaller I don't know if this will work.

Do any of you know if these other widths will work without changing the rim?

 

There is a recommened range of rim sizes for each size tire. The recommened size relationships do vary a bit between manufacturers so once you find the tire you want, verify the manufacturer recommended rim sizes for that particular tire. The closer you can get to matching the exact rim width to the exact footprint width the closer the tire will perform to design specification. You also need to verify the backspacing on the stock Prius to maintain the distance from the fender to the wheel so it doesn't look funny.

In my case I was severely limited in my wheel selection due to a 6 1/2 inch backspace on my wheels.

Also, you want to verify the correct fit when you finally are ready to install your new wheels. The first set I ordered in, although the correct size, was just a bit "sloppy" so they were sent back and the replacement set worked just fine.

It is very difficult to think like an engineer in matching all the sizes and performance factors, as well as trying to improve the looks of the vehicle at the same time. Something so simple as changing tires and wheels becomes a full scale research project. Different tread patterns even have different sound characteristics as well and that matters a lot to some buyers.