What's the performance difference between 15 inch, 16 inch and 17 inch tires (all else equal)?

[dpb]

On 07/19/2017 10:45 AM, Mad Roger wrote: ...

If all else were equal, yes...but as others note, it isn't all equal in reality in virtually all real-world cases.

Even their test timing data reveals that -- since F=ma and m is constant for the same test vehicle the acceleration is a=F/m or again to first approximation a constant for each torque setting. Hence, having run the timing test for a 15" --> 7.6 sec and 19" --> 7.9 sec, the direction of change is the expected of being slower but if were the only effect and the approximation were true then the ratio of times should be the same as the radius/diameter ratio and it ain't even close --

19/16 --> 1.19 where as 7.9/7.6 --> 1.04.

I'm guessing the same effect will show up in the highway mileage results if one were to test it.

[trader_4]

I don't see the decrease in fuel economy. Just because a car accelerates slower doesn't mean that it will use more fuel. I think there is a difference in rolling resistance between tires of different diameters, but as said previously, when going to larger diameter wheels, the tires other characteristics, eg profile change too. That would have an effect on rolling resistance.

Again, torque does not translate directly into fuel economy. I could apply less torque over a longer time period, get to the same speed and not necessarily use more fuel.

[Roy Tremblay]

Maybe I read the article wrong that was referenced by Ed Pawlowski?

I understand your intuition, which is the same intuition we all have but if it's true what that article from Ed says, then 25% less torque at the contact patch means 25% less torque to combat increased wind resistance at highway speeds.

That means, in the words of the article, the engine has to 'work harder' to combat that wind resistance.

You tell me how making the engine work (presumably a lot) harder increases fuel efficiency.

Neither of the two articles mentioned that factor.

And unsprung weight goes up by a few pounds

That's a fair assessment since 25% less torque for a one-inch increase in diameter is astounding.

Can you take a look at the article to see if I did my math wrong?

360 Newtons is 78% of 460 Newtons. 460 Newtons is 128% of 360 Newtons.

That's roughly 1/4 if my math is right. (I never know which direction to quote but both end up being about 1/4.)

[trader_4]

Look at the energy required to lift a 100 lb rock two feet. I can do it two ways, with a 2ft lever that provides low torque, or a 10 ft lever that applies higher torque. Which takes more *energy* to lift the rock? Answer: they both take the same amount. Now apply that to the fuel economy issue.

Or take the example of a bicycle with different gear ratios. While the ratios vary, and with some ratios you couldn't get up a hill, while with other ratios it's possible or even easy, it doesn't change the energy input required.

[Ed Pawlowski]

There are little to no performance gains for any of the above. It is all about looks. Duel exhausts? Ever look at the pipes underneath? They may end up with two termination points but that is after it all goes through the same single pipe. Duals go back to the V-8s in the

1950s.

A car manufacturer sends me (and many others) an occasional survey. They may ask what I think looks better. Two pipes on opposing sides versus two pipes next to each other on one side versus two pipes in the center. They never ask what I think of performance, mostly style once in a while ergonomics on control location but that is for style of the dashboard.

Its all about style. If people don't like the looks they won't go to the showroom.

[Mad Roger]

I will agree with you that most people say that you get better gas mileage, but we haven't seen a reliable article that says that yet.

All we've seen is the two articles that say nothing good performance-wise will come out of increasing the overall diameter.

I understand your intuitive argument that one spin of the wheel is a few inches more but if that one spin comes at a cost in the engine working harder, then we may not get the economy we intuit.

The wind resistance has to be overcome. Torque isn't only for starting at a dead stop. Torque is also needed to overcome wind resistance (which gets appreciable at speed).

I did not consider when I originally asked that there are two situations:

• Larger wheels with lower profile tires resulting in the same diameter

• Larger wheels resulting in a larger diameter

Maybe. Maybe not. It's fair to ask whether fuel economy is increased when the overall diameter of the tire-and-wheel assembly is increased.

So far, neither of the two articles has said that.

I agree with your intuition - but our intuition isn't good enough for a correct answer. We need to find a reference that reliably backs up our intuition.

I guess what you're saying is that everything is relative. I think we all intuit that larger diameters mean fewer inches per revolution which we associate with overdrive gears, which, to our intuition, mean better gas mileage at cruising speed.

However ..........

Cruising speed isn't free. You still have to overcome wind resistance (which is appreciable).

What overcomes wind resistance? Torque at the contact patch, right?

If that's what overcomes wind resistance, and if the engine has to work harder to achieve that torque to overcome the wind resistance, then we may not get the gas mileage we intuit.

I don't know the answer. I only know what the two articles said.

[Mad Roger]

I tried to phrase the original question to keep compensation out of the picture because with compensation, anything can change.

I just wanted to know more about the physics of larger overall diameters.

So far, no reference has given a single good performance impact that results from just the one change of diameter.

[trader_4]

Why would that be? I see lots of things done for styling and eye appeal that are just for that purpose.

How many people looking at a stylish SUV think that those wheels look like they steer better?

Who ever said that it did?

[Mad Roger]

At cruising speed, say 60mph, the main component of force is the drag coefficient, isn't it?

So you need torque at the wheels sufficient to overcome the wind resistance, even if you maintain a set speed of 60mph.

If the needed torque to maintain that speed of 60mph is 100 Newtons (plucking a number out of thin air), and your engine has to work harder to generate a torque of 100 Newtons, then we may not get the fuel economy that we thought we'd get.

Rest assured that my intuition is the same as your intuition, which is that we "should" get better fuel economy by gaining an inch on every rotation of the tires, but intuition isn't always correct.

It would be nice to see a reliable article that backs up our intuition.

It will be nice to find a reliable article that backs up our intuition.

It's a direct quote from the article so I can't argue it better than the authors did themselves.

[trader_4]

I never said that most people say that you get better gas mileage. I have no idea what most people would think.

I didn't say that either, in fact I said the opposite. You're looking at torque, not energy expended. If I lift a 100 lb rock with a 2ft lever or with a 10 ft lever, one involves 5 times the torque, but the energy used is exactly the same.

Then you must not pay much attention to all those wheels out there.

No, it definitely does not, per the rock example.

It's fair to ask whether fuel economy is increased when

Clearly you don't agree with my intuition, which is backed with physics.

[Mad Roger]

I'm looking for anything I can find that tells us what "really" happens to highway fuel mileage with a one-inch overall increase in tire-and-wheel diameter (and no other changes).

This is a physic forum which says that you need more energy with a larger diameter, all else being equal.

"attempting to spin a 2x larger wheel at the same speed would be attempting to move the car 2x faster. Since energy is force x distance, traveling 2x faster for 1 hour necessarily requires at least 2x more energy (since you covered 2x more distance)....

Even more bad news is that in the real world, air resistance is actually a square function! (2x more speed means 4x more air resistance!). This means you would actually probably need at least 4x more energy..."

[Mad Roger]

Maybe I'm wrong that people try to replicate an image of speed and handling (e.g., why do people put M3 badges on a non-M3 bimmer then?).

Anyway, I just want to know what the performance impact is of a larger diameter change of one inch.

This Car and Driver article tries to answer the question: Effects of Upsized Wheels and Tires Tested

"What?s immediately apparent from the results is that as the wheel-and-tire packages get larger and heavier, acceleration and fuel economy suffer. Neither is a huge surprise, but we measured a 10-percent drop in fuel economy and a four-percent degradation in 0-to-60-mph acceleration from the

15s to the 19s"

[trader_4]

And there is where your physics goes wrong. Torque is not measured in Newtons, it's in Newton Meters. You're completely neglecting the distance part. Sure, the engine applies less torque to the wheels with a smaller diameter tire, but it also has to apply the force through a longer distance. With a large tire, it's more force but through a smaller distance around the tire. With a small tire, it's less force but you have to go more than the one revolution you did with the large tire to move the car the same distance. The

*energy"* is the same.

Now there could be other difference between those tires that affect the rolling resistance, etc, but the basic torque physics are as above.

What you're arguing is like saying that the energy required to move a bicycle up a hill is different, that you can do it with less energy input, based on the gear ratio. There is no free lunch.

[Mad Roger]

The problem is that the math is non linear. The physics forum said it this way.

"attempting to spin a 2x larger wheel at the same speed would be attempting to move the car 2x faster. Since energy is force x distance, traveling 2x faster for 1 hour necessarily requires at least 2x more energy (since you covered 2x more distance)...."

The physics forum said otherwise.

"Even more bad news is that in the real world, air resistance is actually a square function! (2x more speed means 4x more air resistance!). This means you would actually probably need at least 4x more energy..."

I am not a physicist so I'm just looking for the answers like everyone else where my intuition isn't good enough to answer the question correctly.

[trader_4]

The problem is that you don't understand the physics.

Now you're just deliberately throwing obfuscation into the problem. Sure there is more wind resistance at higher speeds. But you were talking about a car moving at the *same speed* down the highway and saying that it involves more or less energy because of differing torques. The bicycle example still applies. The bicycle moving at the same speed will need the same *energy* input to get up a hill but with different gear ratios, the torque applied at the peddles will be different. You apply less torque over a greater peddle movement distance or higher torque over a shorter distance, but the energy input is the same. It's physics 101.

[Mad Roger]

Thank you for pointing out my error of the 25% loss in torque being from only a 1 inch change.

I misinterpreted the Ed Pawlowski article.

The 25% loss of torque came with a four inch change in diameter!

But it's still a loss in torque. The Car and Driver article said the fuel mileage suffered also.

Effects of Upsized Wheels and Tires Tested

"What's immediately apparent from the results is that as the wheel-and-tire packages get larger and heavier, acceleration and fuel economy suffer. Neither is a huge surprise, but we measured a 10-percent drop in fuel economy and a four-percent degradation in 0-to-60-mph acceleration from the

15s to the 19s"

[Tekkie®]

rbowman posted for all of us...

I think this guy is the same argumentative/troll about weight weights and Horror fright bead breakers, etc. He just changed hosts.

[Mad Roger]

I am not going to disagree with you.

That's why I want a reliable reference that backs up any claim of better gas mileage with a larger diameter tire.

This article from Cooper Tire says that our intuition is wrong for example. Do larger tires improve gas mileage?

"The common belief is that a tire with a larger diameter will cover more ground per revolution, thus reducing overall gearing and enabling the engine to run at lower revolutions per minute (RPM). People generally associate lower RMP with less fuel used to travel the same distance. While this may be true in a few instances, it is not the case for most vehicles. There are too many variables to make a universal statement that larger tires are more fuel efficient."

They then explain that the torque curve is critical, which makes the equation too complex for a Usenet discussion instantly (because of the huge number of variables involved).

Huh? You misinterpreted what I said versus what was quoted but that doesn't really matter since all we need is a reliable reference that backs up both our intuition that the larger diameter setup will result in better fuel economy.

So far every reference I can find (e.g., Car & Driver & Cooper/Dunn Tire and the Physics Forum) say that is not the case.

So let's just look for a reliable reference that actually claims that better gas mileage results from just changing overall diameter.

Here's the best I can find, from that Dunn/Cooper reference above: "In a nutshell, the vehicles which stand to benefit from a larger diameter tire are those that produce more torque than they need to in order to maintain speed."

However, it concludes, probably correctly, that... "There are just too many factors to consider to make general statements that larger or smaller tires are more fuel efficient."

[Mad Roger]

We both agree that our intuition tells us that the larger diameter results in better gas mileage.

Yet neither one of us can find a reliable reference that backs up that claim.

Here's an article on "Tire Size Vs. Gas Mileage"

"The most popular notion about how tire size affects fuel economy is that a tire with a larger diameter will cover more ground per revolution, thus reducing overall gearing and causing the engine to run at lower revolutions per minute (RPM). Some assume lower RPM equates with less fuel used to travel the same distance. Though this idea does have its merits and is true in some cases, the effect is hardly universal and varies greatly depending upon the vehicle and the increase or decrease in diameter."

That article concludes the same thing that the Cooper Tire article did, which is "The only vehicles which stand to benefit from a larger diameter tire are those produce more torque than they need to in order to maintain speed."

Both articles mentioned that wider & taller tires reduce fuel economy but let's keep the wider and taller part out of this discussion because it's already too complex with just the one change in diameter since most of us are running on pure intuition.

[rbowman]

I owned a '60 Plymouth which was the bitter end of the fin craze. Plymouth even referred to them as stabilizers:

I never managed to get the car up past 110 and didn't notice any more stabilizing effect than my '65 Dodge which was back to the basic 3 box model:

Style is style. I did go to an outlaw kart race a couple of weeks ago and found out the real use for the wings -- when they roll the wind up on their side with the wing preventing them from going over completely.