Tires: Low Rolling Resistance

[jazz4cash]

UPDATE: These tires give me significantly worse gas mileage than the Toyo Ultras did! The average of the last five tankfuls with the old tires: 40.84 MPG. The average of the first five tankfuls with the new tires: 36.98 MPG.

We drive 15,000 miles per year, and use 38 more gallons per year, or, at $3/gallon, $114.

I guess it's not worth taking the tires back.

Hmm...pretty tough to compare rolling resistance head to head. New tires would generally have higher RR without regard to design. Personally, driving on new tires always makes me want to drive harder because the handling is sooo much better, so my lead foot would negate any design improvement as well.

I DO think you should return the tires and demand a full refund...it would be very entertaining for some of us to hear more of this endeavour and you never know it just might work!

 

[Free To Canoe]

I like my new Toyos.

I am surprised that the nitrogen thing just refuses to die / go away.
I guess most people don't know that air is mostly nitrogen?

Free

 

[Frugality_of_Apathy]

If you wanted the optimal in efficiency You'd want to get your low rolling resistance tires and then have the driving tires siped for better initial friction wasting less energy on acceleration.

 

[haha]

If the tires only last 50k miles but their traction keeps DW out of a wreck I'm a happy camper.

This is how I look at it too. Until I moved to the city where I drive very little, I ran high quality soft rubber Blizzaks in winter, and good pure performance tires in summer. They don't last as long, but they stop in rain, and corner. Tires are the only part of a car that contact the road. Now I have only a little storage space, so I am going with Bridgestone All-Season year 'round. They are OK, but the difference is apparent.

Ha

 

[cantlogin]

...Tires are the only part of a car that contact the road...
Ha

In a perfect world.

 

[ERD50]

The average of the last five tankfuls with the old tires: 40.84 MPG. The average of the first five tankfuls with the new tires: 36.98 MPG.

It is hard for me to believe that (properly installed/inflated) tires could be responsible for ~ 4MPG, OR OVER 10% MPG delta. Just not adding up for me.

Something else is at play here. Q: do the tires have the same rolling circumference as the old? If it is different, a 'mile' might not be registering as a 'mile' anymore. I don't even think that would account for it, but add in driving variables, different seasonal fuel mixtures, etc....

-ERD50

 

[ls99]

New tires always give lower gas mileage: the thicker treads squirm more, dissipating energy as heat. Best mileage is from nearly worn out tires, in spite of somewhat smaller diameter. Add in higher density air in winter, your mileage will be a lot lower.
As usual YMMV

 

[haha]

In a perfect world.

LOL! Don't I know that.

Ha

 

[TromboneAl]

New tires always give lower gas mileage

That's encouraging, maybe that's it.

It is hard for me to believe that (properly installed/inflated) tires could be responsible for ~ 4MPG, OR OVER 10% MPG delta. Just not adding up for me.

Yeah, I know what you mean. I waited five tankfuls to make sure it wasn't just differences in pumps, etc. I checked that the tires are inflated to their max (44 psi).

Q: do the tires have the same rolling circumference as the old?

As far as I know.

 

[NW-Bound]

Talking about gas mileage, I observed the following.

1) Repeatedly driving a round trip from an elevation of 1200 ft to 7000ft, then back, at speeds of 55 to 65MPH, I consistently get 24MPG.

2) Driving on flat land at 1200 ft at the same speed range, I consistently get 22.5 MPG.

Now, considering that going down hill won't get back all the energy expended in going up hill, the above results were counterintuitive to me.

The only explanation I could come up with was that the engine efficiency does not vary much with air density, but the aerodrag is significantly lower at the higher elevation.

Comments?

 

[ls99]

Talking about gas mileage, I observed the following.

1) Repeatedly driving a round trip from an elevation of 1200 ft to 7000ft, then back, at speeds of 55 to 65MPH, I consistently get 24MPG.

2) Driving on flat land at 1200 ft at the same speed range, I consistently get 22.5 MPG.

Now, considering that going down hill won't get back all the energy expended in going up hill, the above results were counterintuitive to me.

The only explanation I could come up with was that the engine efficiency does not vary much with air density, but the aerodrag is significantly lower at the higher elevation.

Comments?

Can't explain the differences in mileage, however the lower the altitude, the higher the density of air. Modern engines have altitude (air density) correction programmed, constantly adjusting air/fuel ratio to maintain 14.7 A/F ratio, except under full throttle, when they generally switch to 11 A/F ratio for power. (more fuel)

 

[NW-Bound]

... however the lower the altitude, the higher the density of air...

Yes. And aerodrag is proportional to air density.

By the way, about aerodrag being THE most significant factor, everything else being equal, meaning engine size and car weight, I also observed the following.

Coasting down a 6% hill, my minivan speed will creep up to 80MPH+, where I have to apply braking. No telling how higher it would go.

Coasting down the same hill, my SUV did not get much more than 70MPH. No wonder it is a gas guzzler!

In any event, the mileage that T-Al got is impressive. I would be happy to get 37MPG.

 

[ERD50]

Wait a minute - isn't gas like 52 cents a gallon or something now? Cheaper than water! Who cares about MPG!


-ERD50

 

[UncleHoney]

That's encouraging, maybe that's it.



Yeah, I know what you mean. I waited five tankfuls to make sure it wasn't just differences in pumps, etc. I checked that the tires are inflated to their max (44 psi).



As far as I know.

Al

I think you need to rethink your inflation pressure. Increased tire pressure may help fuel economy a little bit but it will also increase the effective diameter of the tire and throw the speedometer and odometer off, especially at lower speeds.

According to Michelin the correct tire for your Echo is P175/65R14 I think.

They show at 45 mph that tire should rotate at 897 revolutions per mile. That gives you an effective diameter of 22.48 inches (because the tire flattens as it rolls. If you increase the inflation pressure and in effect increase the effective diameter of the tire to say 23 inches, then the tire will only rotate 877 rotations per mile. That is about a 2.2% change in distance traveled.

It would be interesting to park the car on a flat surface and then alter the tire pressure and measure the change in bumper height and see how much it varies. Any significant change is going to effect your mileage calculations, especially at lower speeds. At high speeds tires will increase in size and there is no data from Michelin so it's hard say what will result.

I think tire over inflation is way over rated. Harder tires may get you a few more miles per gallon but it's also hard on the tires as far as resistance to road hazards and tread wear. Plus it's hard on bearings and suspension components which may lead to shorter component life down the road. And you loose a lot of the performance as far as wet weather traction and braking.

Symmetry®: Browse Tires in Passenger Car/Minivan: MICHELIN® Tires: Michelin Tires

 

[TromboneAl]

I'll look into that, thanks.

I noticed a significant increase in MPG when we went from 35 psi to 44. What you said about tire diameter means that the increase was even higher than measured since we were traveling further than the odometer thought.

The Toyos lasted longer than their expected miles -- that's the first time that's ever happened for me.

 

[SteveL]

Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change. The tires do not "stretch" when inflated. Lower air pressure means more tire is in contact with the road surface, creating more resistance, taking more energy to be moved, generating more heat.

 

[ls99]

Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change. The tires do not "stretch" when inflated. Lower air pressure means more tire is in contact with the road surface, creating more resistance, taking more energy to be moved, generating more heat.

I don't feel like it, but an inquisitive character can always do empirical determination: Set tire pressure to 10 psi. Measure diameter. Increase pressure in 5 psi increments. Graph and publish results.

The contact area does definitely change with pressure. Most tire mfg. have photos of such area on their web site.

 

[cantlogin]

I don't feel like it, but an inquisitive character can always do empirical determination: Set tire pressure to 10 psi. Measure diameter. Increase pressure in 5 psi increments. Graph and publish results.

The contact area does definitely change with pressure. Most tire mfg. have photos of such area on their web site.

Yeah, confused me at first too. The circumference is only directly related to the diameter of a CIRCLE. A flat or flatter tire isn’t completely round but the circumference is the same. It may be that a tire does stretch its circumference when it’s over inflated. I know that tires are said to “grow” at high speeds.

 

[ls99]

Yeah, confused me at first too. The circumference is only directly related to the diameter of a CIRCLE. A flat or flatter tire isn’t completely round but the circumference is the same. It may be that a tire does stretch its circumference when it’s over inflated. I know that tires are said to “grow” at high speeds.

Ok. Jack up the wheel, so tire is not weight bearing.
Rinse, repeat pressure tests from 20 psi.

 

[Nords]

Ok. Jack up the wheel, so tire is not weight bearing.
Rinse, repeat pressure tests from 20 psi.

Maybe we should transfer this post to the [-]But what will I DO all day?!?[/-] "What did you do today?" thread...

 

[FinanceDude]

Thanks guys, and thanks, Honey. The tires are ordered and on their way.

My only regret is that I didn't think to name this thread:

Time to Retire
​

 

[TromboneAl]

Don't see how tire pressure changes tire circumference. Yes, more of the tire is on the ground and the height is lower, but the circumference of the tire doesn't change.

True, the circumference may not change, but the effective radius changes. For one revolution of the tire on the left, the car will go farther than for the tire on the right.

 

[ERD50]

Ok. Jack up the wheel, so tire is not weight bearing.
Rinse, repeat pressure tests from 20 psi.

See T-Al's excellent diagram above. The weight-bearing part of it is essential to the discussion. I doubt you will see much difference with no weight on it.

Since we drive our cars with the wheels on the ground, that is the only measurement that matters anyhow. Measuring center of hub to ground at the low and high pressures will give the delta in radius. [-]Pi * R* R[/-] (edit - correction) 2*Pi*R will give the effective circumference using the hub to ground radius. I'm not motivated enough to do it though.

-ERD50

 

[haha]

See T-Al's excellent diagram above. The weight-bearing part of it is essential to the discussion. I doubt you will see much difference with no weight on it.

Since we drive our cars with the wheels on the ground, that is the only measurement that matters anyhow. Measuring center of hub to ground at the low and high pressures will give the delta in radius. Pi * R* R will give the effective circumference using the hub to ground radius. I'm not motivated enough to do it though.

-ERD50

I am not sure that this is relevant. Unless we find a way to break traction, the ciircumference is what matters, not the radius. The formula given above is incorrect even for an abstract perfect circle. However, to model a tire in use on an automobile we need to use circumference as physically measured around the tire. We can visualize it accurately as a bulldozer track. Absent traction breaks, it seems clear that an inch of tire must traverse an inch of pavement.

ha

 

[FUEGO]

See T-Al's excellent diagram above. The weight-bearing part of it is essential to the discussion. I doubt you will see much difference with no weight on it.

Since we drive our cars with the wheels on the ground, that is the only measurement that matters anyhow. Measuring center of hub to ground at the low and high pressures will give the delta in radius. Pi * R* R will give the effective circumference using the hub to ground radius. I'm not motivated enough to do it though.

-ERD50

Isn't the circumference 2 * pi * R? Or are we talking about different measurements? Pi R^2 is area.