Old vs new tyres
 

Considering the same brand of tyre, the only variable being new vs old, what do you people think would offer improved FE at highway speeds?

I'm guessing it's a question of decreased tyre circumference with the old tyres (which affects gearing and hence engine rpms), with one 'positive' of old tyres being that the tread is shallower and hence possibly more aero (I know, I'm treading on shaky ground there mentioning that, with all the safety implications - note that it hasn't rained on this island for probably 6 months or so)

Of course degradation of the rubber compound would also play a part. But does this improve Crr or what?

Food for thought.

CRR is improved as the tire wears. Tire diameter difference is IMO splitting hairs.

You would save more gas if there was no wind.
you would save more gas removing your mirrors.
There is no measurable difference in size.
There is no measurable difference in 'aero' or smoothness.

IMHO I would alwys want the newest tire possible for a HOST of reasons that have nothing to do with the reasons you list.
You are reaching for the 'fruit at the very top of the 30ft tree'.
Stick to the low hanging stuff.

But if you want to think about it.....
There is only an 'optimal' point in tire wear. And that is probably at 1/2 tread. And the trade off would be so slim as to not even register.
Yeah, a slick tire is aerodynamic......so? The chance of an accident has increased exponentially.

It's useless for driving.

You mean the Crr value decreases for an old tyre?

Yes! See these earlier postings:

• http://ecomodder.com/forum/showthrea...sts-23414.html

• http://ecomodder.com/forum/showthrea...-fe-23413.html

Braking distance is severely affected by tread wear, a worn tire takes 50 to a 100% longer distance to come to a panic stop then a new tire.
I'll take the braking ability of a new tire over the RR of a worn tire anytime.

Tire rolling resistance reaches its minimum value in the first 5000 miles on passenger car tires. After that the sidewalls and belts loose their stiffness which increases the rolling resistance.

Do you have a link that substantiates your claim (not that I disagree with it)?

Do you have a link that substantiates your claim?

To the contrary, I've heard that slicks improve road performance on dry, solid surfaces. I'm to believe that tread exists for loose or wet surfaces, and that is why slicks are used when racing.

Do you have a link that substantiates your claim? This one I highly doubt. Your claim is that stopping distance is doubled for a worn tire, which doesn't seem possible.

Do you have a link that substantiates your claim?

Barry's Tire Tech

(That site is maintained by an EcoModder member & tire engineer.)

Thank you Metro.

I only repeated my question to everyone because this site should have substantiated claims instead of pages worth of speculation.

It was my understanding that FE generally increases as a tire wears, and your link suggests the same.

Here is what I found from Tirerack:
The page also goes on to discuss how worn tires affect the odometer reading. On a test car fitted with 205/55R16 tires, a 100 mile test showed a 1.5% difference in odometer distance with worn tires when compared with new tires. This would directly affect the accuracy of MPG calculations taken from the odometer or trip meters.

On a 30mpg car, new tires might actually decrease FE by 1mpg, and the change in odometer reading might show another 0.5mpg "loss".

Kinda hard to argue with 10,XXX,XXX tires being sold With tread........

But hey, let's start saying how everybody needs to be driving on donut spares.....

...or Police Cars driving on spare donuts?

Thanks for the additional info!

I'm having to remove my foot from my mouth after looking into your claim and finding evidence to support it.

The link shows a test in which a passenger car and truck took nearly twice as long to stop from 70mph with worn out tires when compared to new.

That brings us to Mcrews post showing that tread is clearly thought to be important. Even if one lived in Death Valley, having tread just for the possibility of encountering infrequent rain more than justifies the marginal loss in performance and wear the other 99.9% of the time.

Tread is often unnecessary, but essential to safety some of the time.

I have 269,000 miles on my 2002 Q45 Sport.
I replace the Kuhmos every 40-44k miles.
From personal experience alone, I can tell you that worn tires suck.
I'm just not going to trade .001 better mpg for the risk.
Pump up to 45 psi and use nitrogen. And run tires w/ tread.

ANd buy a scangauge.

More like 0.7mpg for your vehicle, but I get your point :thumbup:

Nitrogen is an unnecessary expense if you check your tire pressure regularly.

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It's worth emphasizing that the stopping distance comparison that Tire Rack did was in wet conditions only. They didn't test in the dry. They also artificially "aged" the shaved tires by baking them in an industrial oven for several weeks.

So the idea that dry traction may be unaffected (or even improved) by reduced tread depth still holds.

No argument there. Just clarifying their test.

Tire Test Results : What Honest Abe Doesn't Tell You About Minimum Tread Depths: Part 2 - Dry Braking

Part 2 is dry testing, but only the car, brand new vs 4/32 & 2/32, all stopped within 1.5 feet of each other 50 mph to 0. And worn tires corner better, which kind of tells me the cooking in the oven didn't affect the rubber much.

So during the summer, newer tires on front, wear out old on rear, front's do most of the braking, wet or dry and I'm not cornering that fast, at least when wet.

I remember seeing a chart of the stopping comparison many years ago and it stuck, I cannot find that particular article or chart but did come across the video companion to the link that " redpoint5" has posted, from the tire rack.
http://www.youtube.com/watch?v=zA6MUlVNkLM

I am actually surprised that a worn tire performs better on dry roads. I always assumed the rubber compound in road tires would not grip as the race slicks do.

...which is *why* drag slicks are, well, slick = no tread, just ALL surface contacting rubber.

My worn out tires are not good in the rain, but they won the low speed coast down test. I guess that would apply to bald insight tires compared to new H rated tires also.

An old thread, but I had to affirm the truth of redpoint5's post. Often, my tires show an odometer error of plus 3%, which I use in my MPG calculations. As the tire wears, I have to reduce the error to 2% & sometimes to 1%.

Here's ecomodded's post, well worth repeating about worn tire braking distances on wet roads.

I changed out a set of worn Yokohama Avids for a new set of Bridgestone Ecopias a few months ago and was surprised that the new tires provided at least as good mileage as the worn set.

It was probably due to the Ecopias having a better rolling resistance factor, even though they had 10/32" treads to squirm around on compared with less than half that on the old Avids.

There was one other possible influencing factor: the old tires were 20560R16 and the new ones are 20565R16. So the new tires are several percentage points taller, which made it necessary to adjust my odometer readings because the odometer became measurably "off" by the added territory each tire revolution covered.

It is also possible the new taller size will yield a MPG advantage due to a lower engine RPM count per mile. I might be able to see this with time, although the difference may be too small to claim.

Since I began driving with used taller & wider tires, I get better MPG, despite the tires having wider tread. It is wonderful that MPG goes up due to fewer rpms, despite heavier, wider tread. Of course, the wider tread & taller tires, now give my small Hyundai Accent, better cornering, a bigger feel on the road & makes it more highway viable. Some of my used tires are 8% to 9% taller & rpms have taken a big whack downward. Your tires appear to have 3% height difference. I might think you could even go with a taller tire yet, maybe another 4%.

One disadvantage of my tires, tho: My tires are too tall now, to give snow chains clearance, & keep them away from mudflaps & wheel wells. For regions needing winter snow chains, smaller tires need to be obtained.

One safety factor for driving used tires: I change my used tires with more tread than I changed my "new" tires, because used tires are 4 times(more?) cheaper than new tires.

Yes, the taller tires do reduce engine RPM, which reduces internal friction by reducing how many times the revolving parts have to turn over for each mile. Up to a point, this reduced RPM factor will increase MPGs, especially on the highway.

However, a point of diminishing or reversed returns can be reached by causing the engine to labor too much if / when the tires (effectively, the gearing) become too tall to handle. Only by trial-and-error can one find that "cry uncle" point, as each car is operated in different environments and circumstances: weight, wind, hills, etc.

Another thing: the clearances for the taller tires have to be watched. It is possible that even though a car can benefit from a taller tire based on the factors above, it may not have enough clearance in the wheel housing to take advantage of a taller tire. This also takes trial-and-error to find out. There is a point that the ever-larger tires start hitting the fenders from suspension flex, and a point that the front wheels can't make sharp turns because the tires are hitting things there.

I'm glad I got the larger "65" aspect tires. They seem to be justifying the switch for me.

It's really not all that hard to make a cardboard profile tire to check clearance. Get a rim that has proper offset, cut a chunk of cardboard that fits down in the rim 90 degrees to direstion of rotation. Where it hits on the body, take a measurement. That is the biggest tire you can fit. Make sure the axle is supported in the proper configuration. You should probably subtract a couple of inches for suspension travel, but I have seen people who constantly scuff their tires.

Many older small cars have their engine rpms way too high, at highway speeds. Taller tires will never reach "dimiinishing" returns. On my motorcycle once, I changed the "sprocket/counter sprocket set" ratios by 15%. My MPG went up 10% into the 75+MPG region. Yet, I was able to accelerate past the designed "top speed" of the cycle, well into triple digits. I think the cycle would have continued to accelerate, till it destroyed itself...... & me, with it.

AND the "bigger feel" really does work, which I mentioned somewhere else. With the Hyundai Accent, I traveled to eastern Washington from western Washington for a sweet 600 mile day trip. Normally I use the bigger Elantra, when driving longer faster distances. But the bigger tires was really nice, gobbling up 600 non-fatiguing miles. Just really sweet.