Tires. Skinny/wide? Tall/low pro?

[CapriRacer]

Originally Posted by elhigh

...... I generally think taller/narrower is the way to go, other factors being equal.

Ah ..... Mmmmm ...... not exactly.

The data seems to point to wider and taller being better. See my web page on the subject:

Barry's Tire tech - Rolling Resistance and Fuel Economy

The best guess as to why that is is that the increase in load carrying capacity causes a greater reduction in RR than the effect the smaller volume of material.

But, again, a tire size change is so small a change if everything else is equal, that it's hardly worth mentioning! Differences in the make and model of tire are SOOOO much larger.

[ksa8907]

Originally Posted by elhigh

I'm not sure why you're concerned about the tires' load rating, on a Civic Si virtually anything that comes up is going to easily cover the car's max GVWR.

If you're willing to order online, Tire Rack has a 185/60-16 for a 24.7" tall tire that is a skosh narrower than the 195. It has a AA traction rating and a 240 treadlife rating, suggesting it's a fairly hard compound.

I generally think taller/narrower is the way to go, other factors being equal.

If I'm not mistaken, AA would indicate high traction and a 240 wear rating would indicate relatively low tread life.

[Ecky]

Originally Posted by CapriRacer

Ah ..... Mmmmm ...... not exactly.

The data seems to point to wider and taller being better. See my web page on the subject:

Barry's Tire tech - Rolling Resistance and Fuel Economy

The best guess as to why that is is that the increase in load carrying capacity causes a greater reduction in RR than the effect the smaller volume of material.

But, again, a tire size change is so small a change if everything else is equal, that it's hardly worth mentioning! Differences in the make and model of tire are SOOOO much larger.

I wonder why it is then, that most of the fuel economy "supercars" (even those made recently) have used very narrow tires with tall aspect ratios? Is it aerodynamics? Surely VW had this data when designing the XL1.

[Ni87]

capriracer would you recommend a 185/55r16 over a 195/50r16? What would you suggest. has the wider and taller tire worked on your own vehicles?

Thanks a lot for you input everyone. I appreciate that. Those Yokohama ascends were actually the tire I was looking at. They seem like the perfect all around tire and I've heard they last a long time. My second choice was the Yokohama vigors which were more of a performance all season but like you said they are designed more for traction vs. Economy. And yeah the Utqg number the higher the number is usually better from what I've read. The higher number generally indicates a longer tread life. From what I've read a tire that has a 200 Utqg number is likely to last twice as long as a tire that has a 100 Utqg.

That's why I think I want to go with the Yokohama avid ascends since the Utqg is about 740-800 depending on what size you get. So they are more geared for longer tread life which comes with less traction which I think they made up the difference with the siping. It is also designed for an eco focus. So that's why I think it's the best tire for me.

As far as my commute it's strictly highway. My home town is roughly 10 hours away so when I go to visit my family I bring the old civic. My goal is to one day do it in one tank. But I need roughly a 20 percent increase in fuel economy so it might be tough. But I think going with the 185/55r16 might be best since it's the skinniest tire out of the three sizes and second tallest. I do think the gear reduction would help. It made a world of a difference in my xterra.

What are some of the best tire setups you have tried and also best mileage increaser. I know the biggest difference comes from driving habits. I am just curios to see what mods helped most too.

Thanks again for all the input. My question is why doesn't having a low profile tire help with economy? Like someone said the trains have steel wheels for efficiency and we have air more for the cushy ride. Wouldn't the lower profile make for a harder more effienct tire?

[Ecky]

Originally Posted by Ni87

Thanks again for all the input. My question is why doesn't having a low profile tire help with economy? Like someone said the trains have steel wheels for efficiency and we have air more for the cushy ride. Wouldn't the lower profile make for a harder more effienct tire?

Maybe it does, maybe not. This isn't data I'm in possession of.

What I can say for certain is that rotating mass is a big deal whenever you have to change speed - 10 extra pounds of weight in your tires has a lot more impact on the amount of fuel it takes to get up to speed, and the amount of brake wear needed to slow you down, than 10lbs in the car. This is because the rotational inertia formula is mass times radius squared.

Or, in other words, tires with larger circumferences have exponentially more inertia than those with smaller diameters, while the rotating speed only goes up linearly. So, it's doubly important for this to get a light weight (narrow) tire if it has a larger circumferences. And for low pro tires, to keep the same wheel diameter they need to have larger diameter rims, so all of the weight of that metal is farther out, meaning exponentially more rotating mass again.

A narrower tire also has less aerodynamic drag, which is highly important at higher speeds.

Whatever the truth is about aspect ratio and width vs rolling resistance, the highest fuel economy cars have all had narrow, lightweight tires.

[oil pan 4]

I would put the tallest tires that wouldn't rub.
I put tires an inch taller on my wife's car. It was very economical. The taller set just happened to be $50 cheaper for the same yoko ascend tires. So before I even put them on they had saved me a fill up and then some.
OE is a 215 tire, I went with a taller 215 tire on that car.

[Ecky]

Originally Posted by oil pan 4

I would put the tallest tires that wouldn't rub.
I put tires an inch taller on my wife's car. It was very economical. The taller set just happened to be $50 cheaper for the same yoko ascend tires. So before I even put them on they had saved me a fill up and then some.
OE is a 215 tire, I went with a taller 215 tire on that car.

Did you adjust the speedo, or is she driving faster?

[oil pan 4]

The speedometer was already 3mph slow, this fixed that.

[RedDevil]

Originally Posted by Ecky

Maybe it does, maybe not. This isn't data I'm in possession of.

What I can say for certain is that rotating mass is a big deal whenever you have to change speed - 10 extra pounds of weight in your tires has a lot more impact on the amount of fuel it takes to get up to speed, and the amount of brake wear needed to slow you down, than 10lbs in the car. This is because the rotational inertia formula is mass times radius squared.

Or, in other words, tires with larger circumferences have exponentially more inertia than those with smaller diameters, while the rotating speed only goes up linearly. So, it's doubly important for this to get a light weight (narrow) tire if it has a larger circumferences. And for low pro tires, to keep the same wheel diameter they need to have larger diameter rims, so all of the weight of that metal is farther out, meaning exponentially more rotating mass again.

A narrower tire also has less aerodynamic drag, which is highly important at higher speeds.

Whatever the truth is about aspect ratio and width vs rolling resistance, the highest fuel economy cars have all had narrow, lightweight tires.

Sorry, but tire size has no effect on rotational inertia.
A taller tire has quadratically (not exponentially) more inertia for its bigger size, but the speed relation is also quadratical - and it rotates less fast than a smaller tire would for the same speed of the car, so these effects precisely cancel each other out.

Tire weight does increase inertia on a linear scale. Also it should be noted where the weight is; the thread adds twice its weight to the inertia as it is at the edge of the circle(*), the axle is just dead weight counting only once.

The biggest power loss from heavy wheels is caused by the suspension having to work harder. The lower the unsprung mass, the easier the wheels can track the road, the less the tire deforms on the bumps.

Taller tires also ride the bumps and troughs in the road surface more gently, which more than compensates for their generally heavier weight.

(*) Inertia of rotational mass.
It takes 1 Newton of force during 1 second to accelerate a mass of 1 kilogram to a speed of 1 meter per second.
During acceleration it moved half a meter, times 1 Newton means it took 0.5 Joule of energy to get it up to speed.

Now it does not matter whether that mass is moving straight on or in a circle. It just takes 0.5 Joule to get it to that speed.
A wheel is both rotating and moving forward at the same speed. So mass at the edge has to be spun up and moved as a whole; so that's twice the power needed.
It takes 1.0 Joule to speed up 1 kilogram of tire thread to a speed of 1 meter per second, half of it needed for spinning it up and the other half for getting it moving as a whole.

[Ecky]

Originally Posted by RedDevil

Sorry, but tire size has no effect on rotational inertia.
A taller tire has quadratically (not exponentially) more inertia for its bigger size, but the speed relation is also quadratical - and it rotates less fast than a smaller tire would for the same speed of the car, so these effects precisely cancel each other out.

Correct me if I'm wrong, but circumference is Pi*diameter(or 2r), whereas inertia is mass * radius (of rotational average center of gravity, or whatever the proper term for that is) squared. So, a wheel with twice the circumference would have twice the radius, and be spinning half as fast. It would have four times the rotational inertia at a given speed, but since it's spinning half as fast, its inertia is only doubled.

What am I missing?