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View Full Version : Does tire size really matter?



Yeadon
04-06-2009, 11:05 PM
My girlfriend took her 2001 Toyota Corolla LE down to a local tire store earlier this evening. She was supposed to get P185/65/R14's (this is the size called for in the manual) ... but the clerk there instead said she should get a set of P195/65/R14's. ... so, she got them.

Now, here's the real question ... generally, it's not a huge deal to get a lower aspect tire, say a 60 versus a 65, but does it really matter if wheel width is an extra 10 millimeters wide?

Seems like there is enough room in the wheel well to swallow these things, though I'm still pretty tempted to take them back tomorrow and ask for the manufacturer's recommended size.

Any informed opinions out there? What are the pros and cons here?

cybulski
04-06-2009, 11:08 PM
if their is nothing the tire can rub while turned all the way, and with the strut compacted, then you should be fine.
if it were a new car you might be concerned about voiding warranties.

Yeadon
04-06-2009, 11:13 PM
if their is nothing the tire can rub while turned all the way, and with the strut compacted, then you should be fine.
if it were a new car you might be concerned about voiding warranties.

Good point.

I would need a very fat man to help me simulate that. But it would be worth the effort.

cat_duh_maran
04-06-2009, 11:15 PM
So the dude at Walmart or whatever tire store you used knows more than the manufacturer? I'd be tempted to take them back unless a more reliable source said there was a real advantage ie. gas milage, handling, etc.
Bruce

brad9798
04-06-2009, 11:16 PM
10 mm is simply that much more rubber on the surface ... and surface contact leads to better stability and traction ... thus, perhaps, more safety!

Bob (oh, THAT Bob)
04-06-2009, 11:22 PM
Should be OK. When I have upped the width, I lower the aspect ratio so that the rolling radius and circumference is the same. This preserves the intended design geometry, the biggest effect on a front-drive car would be scrub radius which will effect steering pull under braking. I went to 195-60 from a 185-65 I think. The car spec originally had 175-70s but for the new 195s, user feedback said they ran very narrow for their size so I upped two sizes and they were right. As I recall, tirerack.com used to list the specs for the tires they sell. Check your speedo and odometer using a measured mile and a stopwatch, if it is way off you might want to return them. Sometimes a larger tire is a good move, but sometimes the tire place has more of them in stock so they push them. A 195 will wear better, have slightly more rolling resistance, but to preserve the same section height you will need a slightly lower section profile, as section height (60, 65) is the percent of section width.

rbgarr
04-06-2009, 11:30 PM
About 3/8" is the total difference in width, isn't it? 3/16" inside and out?
I wouldn't worry but maybe Katherine has an official opinion.

I bet tire width at the widest changes more than that with underinflation.

Lew Barrett
04-06-2009, 11:32 PM
Maybe, but also maybe not! Tire width also adds a bit of resistance at speed, thus potentially marginally less MPG. Go fast: skinny tires, corner fast: fat tires. Probably not noticeable except under exacting conditions. Bigger issue is how they fit the rims. On a bike, swapping different tire sizes onto the same rim is a big bad baddaboom because it changes the profile as well. Since car profiles won't change as radically with small changes in width again it might be within the tolerance of the car, but the wheel is sized for a given tire and vice versa.

In respect to changes in aspect ratio, any change outside of the nominal will affect both the accuracy of the speedometer, and your gearing. It may be minimal, but it will be affected. Ant that change is inversely related to tire size, meaning it has a greater effect on a smaller wheel.

I'd like to size the tires as the manufacturer suggested for that particular wheel.

aspect ratio/speedo chart thingy (http://www.miata.net/garage/tirecalc.html)

wheel/tire calculator thingy (http://www.wheelsmaster.com/rt_specs.jsp)

Note: your Gf's speedo is now different than it was, as is her gearing. Immutable.

htom
04-06-2009, 11:34 PM
Probably not the best match. :( Over a 1% error is going to cause problems with the computers, I would think, both engine and maybe with braking calculations.
http://www.miata.net/garage/tirecalc.html gives tire measurements
http://www.tiresizecalculator.info/ generates a list of "close" tires

S B
04-06-2009, 11:35 PM
About 3/8" is the total difference in width, isn't it? 3/16" inside and out?
I wouldn't worry but maybe Katherine has an official opinion.

I bet tire width at the widest changes more than that with underinflation.
Wider tires are supposed to be better on dry road, but the added surface increases the potential for hydroplaning and floating on snow.

cybulski
04-06-2009, 11:36 PM
yeah it might change the numbers a bit

Yeadon
04-06-2009, 11:38 PM
You guys have me clicking through all sorts of interesting calculating thingies. This is actually pretty interesting.

Lew Barrett
04-06-2009, 11:40 PM
We must have posted together HTom. The Miata calculator (my first reference) is one of my very faves.

Lew Barrett
04-06-2009, 11:41 PM
Oh yeah, she's back to Sears! I'm off to the car store! Hehe. I want the whole chimichanga of wheel holder to go with the wheels! It's OK to make that sort of change as long as you have a reason for doing it. Without the reason, it's not a good idea.

cybulski
04-06-2009, 11:41 PM
i always go to a bigger tire on my vehicles, it usualy takes away from my gas mileage, and maby a little rougher ride,makes my odometer off by a few miles per hr , but i like the extra heighth and width, but i do alot of off roading, my tires are always ate up anyway,

Yeadon
04-06-2009, 11:45 PM
So, here's my gut feeling. I bet the new tires are fine, but when it comes to dealing with tire shops, mechanics, body shops, I like things to be the way the manufacturer recommended.

I think I'll call down in the morning, tell them about the situation, then ask what size tire a 2001 Toyota Corolla LE is really supposed to have.

I looked online, and a 2003 Corolla can take the larger tire ... but the 2001 and 2002 take the smaller tire. If I had to guess, the clerk just fat-fingered the computer and sold the wrong size tires by accident ... or like Bob said above, was trying to push tires they had overstocked. (This would irritate me.)

I'd like to say I don't have time for this, but things are slow around the ranch and I probably do have time for a good phone argument.

cybulski
04-06-2009, 11:47 PM
yeah, if that would make you feel more at ease , it would probly be the smart thing to do, at least you wouldnt worry about it

brad9798
04-07-2009, 12:18 AM
Why is it that folks think that tire guys ... mechanics ... car folks always try to push what is 'bad' for the consumer? :confused:

Paul Girouard
04-07-2009, 12:21 AM
Note: your Gf's speedo is now different than it was, as is her gearing. Immutable.




Well, well Yeadon's in for a exciting night, hopefully her gearing will be racier! Even if it is immutable :D Can we see some photos of her in her new speedo? Huh, can we???

Yeadon
04-07-2009, 12:25 AM
http://www.delnat.com/elements/Tire_markings_sm.gif

You're gonna have to be alone for a few minutes, huh Girouard?

Here's another shot from a different angle ...

http://www.nctint.com/gallery1/bra.gif

Paul Girouard
04-07-2009, 12:35 AM
http://www.delnat.com/elements/Tire_markings_sm.gif

You're gonna have to be alone for a few minutes, huh Girouard?

Here's another shot from a different angle ...

http://www.nctint.com/gallery1/bra.gif





Humm, that was some what disappointing!

Paul Pless
04-07-2009, 06:16 AM
Dude you should be happy and go down and thank the guy at the tire rack for talking your girlfriend outta those bow legged double duece 22's and air shocks she had her heart set on when she walked in the door.... BLING BLING!!!

jack grebe
04-07-2009, 07:08 AM
10 mm is simply that much more rubber on the surface ... and surface contact leads to better stability and traction ... thus, perhaps, more safety!
LMAO, you can't be serious. Wideth has nothing to do with
the amount of "rubber" in contact with the road.
Contact on the road is determined by the weight on the tire and
the PSI of the air in the tire.
PSI X surface contact = total weight.

Try taking 4 pieces of paper and sliding them up
tight to the rubber in contact with the road on
all 4 sides of the tread area. Then measure the
actual square inches in contact with the road.
now measure the air in the tire. multiply those
together and save the #. Do all 4 tires and add
the final tally.......you will have the weight of the car.

wider tires will change nothing.

Paul Pless
04-07-2009, 07:16 AM
where'd you come up with that theory jack?

Keith Wilson
04-07-2009, 09:22 AM
Unless the slightly wider tire rubs on something, it won't make any significant difference IMHO. The change in type of tire and rubber compound will make far more difference than the additional 10mm of width. It might be very slightly better, as Brad says, but it isn't much of a change. I wouldn't worry about it.

P.I. Stazzer-Newt
04-07-2009, 09:31 AM
Did she change ALL the tyres?

Including the spare?

Keith Wilson
04-07-2009, 09:37 AM
Temporary spare (itty-bitty rubber donut).

Popeye
04-07-2009, 09:38 AM
Wideth has nothing to do with the amount of "rubber" in contact with the road.

Contact on the road is determined by the weight on the tire and the PSI of the air in the tire.

PSI X surface contact = total weight.

..measure the actual square inches in contact with the road. now measure the air in the tire. multiply those together and save the #. Do all 4 tires and add the final tally.......you will have the weight of the car.


wtf ???
:confused::cool::rolleyes::eek::D

Gonzalo
04-07-2009, 09:50 AM
The car nuts here at work always go for wider tires for better cornering performance. They'll generally take the widest tires that will fit without rubbing. There don't seem to be negative consequences unless you care about gas mileage, and the car nuts only care about performance. I don't think 10mm will make any difference, as long as they don't rub anywhere. Even gas mileage should not be impacted very much.

Popeye
04-07-2009, 10:02 AM
The car nuts here at work always go for wider tires for better cornering performance. .

not true for wet roads

Noah
04-07-2009, 10:33 AM
What Jack is saying is true - the contact patch is always going to be the same size if you are running the same tire pressure. Don't forget, PSI = Pounds per square inch. You aren't changing the weight of the car or the tire pressure, so it will keep the same contact area. The contact patch will change from a rectangle to a square as you go with a wider tire. For dry pavement this can be a good thing. For wet pavement, snow, etc it can be bad - as the tire rolls over the road it evacuates the water out to the sides of the tire - a square tire has more distance to travel to evacuate the water, and less time to do it.

Then there is the change to the tire diameter that can screw up the Speedo, and anti-lock brakes, etc.

That said, you may not notice any change actually driving the car.

Paul Pless
04-07-2009, 10:49 AM
What Jack is saying is true - the contact patch is always going to be the same size if you are running the same tire pressure.nope

George Roberts
04-07-2009, 10:55 AM
Contact patch size is determined by issues other than the air pressure.

Consider a tire with zero air pressure. It needs infinite contract patch size to support a car.

There are machines that will draw the contract pressure profile between the tire and road. The pressure is not uniform across the surface - even with slicks.

BrianW
04-07-2009, 11:00 AM
Now that this gem of insight has been released on the public, maybe drag racers will stop putting the huge slicks on the back end. :)

Hwyl
04-07-2009, 11:10 AM
Why did this thread go any further than Bob's post

Yeadon
04-07-2009, 11:18 AM
... it offers insight why actual threads about nothing go on for pages at a time ...

Popeye
04-07-2009, 11:29 AM
what jack is saying is true - a square tire has more distance to travel to evacuate the water, and less time to do it.

...

:d

John of Phoenix
04-07-2009, 11:48 AM
Let's say I have a tire footprint that's 6"x6" and 35 psi per tire.
36 sq inches x 35 pounds/sq inch x 4 = 5040 lbs. If the pressure goes down the footprint gets bigger and vice versa. It's plausable at least.

Jack, how'd you get unbanned? Coors Lite?

Say, is there a prize for being the last person to post on this thread?

Popeye
04-07-2009, 11:51 AM
Let's say I have a tire footprint that's 6"x6"

just a surmise , but , wouldn't a 195/65/14 tire have a much smaller footprint ?

Dan McCosh
04-07-2009, 11:54 AM
Let's say I have a tire footprint that's 6"x6" and 35 psi per tire.
36 sq inches x 35 pounds/sq inch x 4 = 5040 lbs. If the pressure goes down the footprint gets bigger and vice versa. It's plausable at least.

Jack, how'd you get unbanned? Coors Lite?

Say, is there a prize for being the last person to post on this thread?


That's not the issue--it's not the same as saying if you widen the tire, the patch remains unchanged. In fact, it is likely to get larger, although the overall construction of the tire could make the patch either larger or smaller. FWIW, the larger tire on the Toyota is unlikely to do any harm, as long as it doesn't rub, and would be likely to last longer and may improve lateral grip.

Might add that a recommended OE tire is recommended for lots of reasons, including maximizing the rated fuel economy. It's not always the best tire for all conditions.

Popeye
04-07-2009, 12:01 PM
i just installed super light weight tire valve caps

man i'm fly'n

John of Phoenix
04-07-2009, 12:01 PM
just a surmise , but , wouldn't a 195/65/14 tire have a much smaller footprint ?

I'm sure it would. I was thinking of my Passat wagon and the numbers are about right at 6x6 and 5000 pounds.

Popeye, are you just trying to get the last post? What's the prize?

Popeye
04-07-2009, 12:14 PM
choosing a marginally wider tire over a narrow tire gets you nowhere

the real answer to the question is found in the type and quality of the tire, not the size

Yeadon
04-07-2009, 01:37 PM
So ... an update ... went out this morning and looked at the tire ... P195/60/R14 ... I'd misread the tire in the dark. Must have been tired. (I'd thought it was a 195/65.)

In other words, Bob was right ... we went to a 195-60 from a 185-65. No big deal. The tire dealership was right on the mark.

I've led you all down a rabbit hole, and I apologize for that. Though I actually found this very educational all around. Hope you did, too.

Paul Pless
04-07-2009, 01:39 PM
merely solidifying your position as a minor bilge troll

:p

Yeadon
04-07-2009, 01:46 PM
Nothing more, nothing less. Just a bit player in a total farce.

Paul Pless
04-07-2009, 01:47 PM
> :D

Popeye
04-07-2009, 01:51 PM
are you going out of town ?

what's your girl friends number ?

Yeadon
04-07-2009, 01:53 PM
weird.

Noah
04-07-2009, 01:57 PM
Ok, this is a tricky question - I did some reading, and it does appear that tire construction has more to do with contact patch area than just weight and pressure, though they are also driving forces. The design of the tire, and how it distributes loads throughout the structure also defines the contact patch size.

My info on Contact Patch characteristics was pretty close though.

From Tirerack.com http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=10

Picture yourself driving along the highway, (just slightly over the limit, well, more than a little) and the sports news comes on the radio. The announcer mentions Shaquille O'Neal. Now, that guy has pretty big feet, but does he put more rubber on the ground with his Reeboks than your tires put on the road ? Hard to believe, but Shaq puts more rubber down than most cars do. The contact patch of most tires is about the size of your hand and has to handle a lot more weight and force than those big Reebok's do.

The shape of a tire's contact patch or "footprint" greatly influences its performance and is dependent on its profile or "aspect ratio". Low profile tires (most performance tires) have a short and wide contact patch that is effective in converting the driver's input into very responsive handling, cornering stability and traction...especially on dry roads.

http://www.tirerack.com/images/tires/tiretech/35_series.gifhttp://www.tirerack.com/images/tires/tiretech/70_series.gif
High profile tires (light truck and most passenger tires) have a long and narrow contact patch which helps to provide predictable handling, a smooth ride and especially good traction in snow.

Popeye
04-07-2009, 02:03 PM
weird.

oops ,

is it a he ?

High C
04-07-2009, 02:11 PM
Everyone is talking about the extra width, but not the extra diameter. The tires she was wrongly sold have a larger diameter than the original. They should be returned for the correct size.

This extra half inch in diameter changes the gear ratios, speedometer/odometer accuracy, and may cause clearance issues.

See all the dimensions here: http://www.miata.net/garage/tirecalc.html

Edited to add: I just saw your latest post, Yeadon, and see that the tire she bought was a 195/60. A look at the tire size chart I linked will show that this size is slightly smaller in diameter than the original. This will effect RPMs and speedo accuracy, though not very much. Decide for yourself if this is a problem. It's a small change, but a change from spec nonetheless.

Henning 4148
04-07-2009, 02:23 PM
Strange. In Germany, if the tire size was not approved by the manufacturer of the car for the rims you got or by an after market manufacturer for after market rims - the whole thing could void your insurance cover.

Isn't this a potential problem in the US as well?

Bob (oh, THAT Bob)
04-07-2009, 03:12 PM
OK, first of all, the general assumptions about force per unit area do apply with tires, sort of. The effect on handling is different though. Tires are non-linear. Meaning, twice as much force per unit area does not yield twice the grip. If you have a few years, I could go deeper. All vehicle dynamics and suspension design starts with tires. You chose the tires for the vehicle and design the suspension and packaging around them.

Regarding hydroplaning: Force per unit area also does not matter. Any increase is small compared to the hydrodynamic pressure of the water trying to lift the tire. It's all about TIME. A longer, more slender tire contact patch gives more time for the water to work it's way around the contact patch before lifting it completely. I know this issue well. Overinflating or underinflating are both bad. A big increase in tire width hurts, but a small increase may be less important than tread design. Modern tires have much better tread design that lets the water pass through the tread before lifting it, assuming the water is not deeper than the tread depth.