Old vs new tyres

[Synchronicity]

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.

[Daox]

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

[mcrews]

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.

[Synchronicity]

Originally Posted by Daox

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

You mean the Crr value decreases for an old tyre?

[gone-ot]

Originally Posted by Synchronicity

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

[ecomodded]

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.

[ConnClark]

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.

[redpoint5]

Originally Posted by Daox

CRR is improved as the tire wears.

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

Originally Posted by mcrews

Yeah, a slick tire is aerodynamic......so? The chance of an accident has increased exponentially.

It's useless for driving.

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.

Originally Posted by ecomodded

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.

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.

Originally Posted by ConnClark

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?

[MetroMPG]

Originally Posted by redpoint5

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

The more the material in a tire - especially in the tread area - the more the rolling resistance.
This means that new tires are going to have more rolling resistance than otherwise identical, but worn out, tires. So when you buy a new set of tires, you should expect a loss in fuel economy.

Barry's Tire Tech

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

[redpoint5]

Originally Posted by MetroMPG

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:

Tire rolling resistance gradually drops by about 20% during the life of a tire as the tread wears from its original molded depth to worn out. This can be attributed to the reduction in tread mass and rubber squirm, as well as subtle hardening of the tread compound during years of service and exposure to the elements.

While this gradual reduction in tire rolling resistance and minor increase in fuel economy may be too subtle to register during the tire's life on a tank-by-tank basis, the virtually instantaneous switch from worn tires to new tires (even if they are the same brand, type and size) will typically result in an increase in rolling resistance of about 20%. Since the automotive industry estimates a 10% increase in tire rolling resistance will result in a 1% to 2% decrease in vehicle fuel economy, drivers should expect to experience a potential 2% to 4% decrease in mpg.

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".