jhumbo

iTrader: (1)

I'm wondering what the effects of a larger tire are on highway gas mileage. There's a ton of factors here, so let's try to limit it a little. Let's assume you're driving an aerodynamic sedan with plenty of power and not some econobox, for example an Acura TL or TSX or something similar. Let's also assume you make no other changes to the vehicle. This is using the exact same wheels/rims and the same brand/style of tire with the same tread pattern. What would the highway gas mileage affect be by going to a tire with larger diameter, perhaps 5% or 10% bigger?

I realize that there would be loss of power, less torque and that city mileage would most likely go down. I'm not interested in any of those, I'm just curious about the effects on highway mileage.

A few things to consider:

• The larger tire weighs more.
• The larger diameter means the car travels further on a single revolution of the wheels.
• At highway speeds, the fuel needed to keep the car moving at constant speed is dominated by the effects of drag and rolling resistances.
• Due to increased rolling resistance, the engine will have to work harder to keep the larger tire moving.
• This extra work the engine has to do will offset to some extent the further distance the car travels per revolution.

I've read a few other car blogs on this topic and I've seen conflicting information. Some folks say that the extra work the engine has to do is greater than the extra distance that the larger tires and will result in a net lower MPG. Other folks say that the extra work is minimal for a car with plenty of power and the extra distance that the tires are travelling will outweigh the impacts of the larger tire.

Hopefully that was somewhat clear. Anyone have any thoughts?

knight rider

iTrader: (1)

Based on your post, I think you know that the answer is "maybe" and "maybe not", depending on the factors you already pointed out.

We could go round and round (no pun intended) about this question, but the best answer is "it depends".

oonowindoo

are you bored at work?
215 tires will always gives better mpg than 275 tires, given everything else stays the same
wheels weights 20 lbs will always give you better mpg than 40 lbs wheels.

the power required for constant rolling speed (Freeway) is a lot less than stop and go (city), so weight is more important than overall size in your everyday driving.
I am talking about going from 17 to 19", not 17" to 24 dubs yo.

So skinnier tires with light weight wheels will give you the best mpg, even though you will have no traction most of the time.

NBP04TL4ME

While I agree with you the OP asked about a larger diameter not a wider width. That is unless that's what he meant.

Mr Marco

You had 666 posts and this is what you decided to post next?

You ARE the devil.

rhettjstone

Lots of variables here. Tires like the Michelin Latitude Tour are designed for lower rolling resistance (baked into the tread compound), regardless of tire size. Remember, though, that any time you go with taller wheels and rims, it's going to throw your speedometer/odometer readings off; software is available to correct that in the car's engine control computer.

WheelMcCoy

With the larger diameter tire, the TPMS sensors will give incorrect readings, which will, over time, lead to a flat tire, and the car won't go anywhere. Original size wheel wins!

OP, by limiting the problem and filtering out much of the real world interactions -- same rims, same cars, highway only -- you are describing a nearly idealized situation. If we do consider your example as a closed system, the conservation of angular momentum goes into effect. Meaning, the angular momentum of the large wheel is the same as the angular momentum of the small wheel. While the behaviors are different -- the large wheel covers more distance while the small wheel spins faster -- their energy is the same. Doesn't matter if the larger wheel is 5% or 10% or 100% bigger.

Think of an figure skater doing a spin. As she pulls in her arms, she speeds up. She doesn't need to expend more energy trying to spin faster.

Now if I just did someone's homework, do not use the first part of my answer. You will get an F.

jhumbo

iTrader: (1)

Hmmmm.... I was hoping for some solid arguments either way. If we nailed down a specific car, do you think we could get some better dialogue? What if we say this is on a 3G TL.

Hmmm... I suppose I was. Yes, yes, I was bored at work.

Correct. I mean tire diameter only. Width of the tire would be exactly the same.

Lol!

So let's throw variables such as tire design out the window. I'm saying that in both cases we'd be rolling on the exact same wheels/rims. In both cases, we'd be rolling on Michelin Latitude Tour tires (or whatever tire you choose to select). Merely a different size.

For example:
235/45/17 vs. 235/50/17 (3.7% bigger)
or
235/45/17 vs. 235/55/17 (7.4% bigger)

jhumbo

iTrader: (1)

So I think I get what you're saying and if I interpreted correctly, you're in the camp of: "Yes, larger tire will improve highway MPG." Right?

WheelMcCoy

Note that the figure skater, when she pulls in her arms and makes herself smaller, doesn't need to use any more energy (gas) to spin faster. Thus, the smaller wheel can keep up with the larger wheel without needing more fuel. So the surprising answer is a larger tire will make no difference in mpg at all (given the initial conditions).

Now add real world concerns like ride comfort, pot holes, and tire flex (larger wheel will have more which will affect cornering), and the question becomes like the commercial that asks how many licks does it take to get to the center of a tootsie pop?

http://www.tootsie.com/gal_licks.php

fuzzy02CLS

iTrader: (2)

Most certainly will decrease gas mileage. You will be using more gas to get the bigger tires moving. Stock 28" on my Explorer to 31's I saw a up to a 4 MPG decrease. To how much will depend on the car.
You also change your gearing slightly unless it's corrected.

fsttyms1

Weight has a huge difference. Using the Michelin Pilot HX MxM4 in the 235/45 vs 55 there is a 4 lb 23/27lb difference in weight. 4lbs doesnt sound like much but, but add that to all 4 wheels it becomes 16lbs. 16 still doesnt sound like much, but its unsprung weight and weight the motor has to turn, that the suspension has to try to control all effecting overall ride.
The same size but in the Goodyear Eagle GT the weights are even more extreme at 235/45 vs 55 at 27/30lbs, using the later, which would be 28lbs difference that can even further the the results.

In my experience in driving well over 1million miles now you could achieve better mileage by increasing your tires PSI 4-8psi higher over the oem "recommended" rating (depending on the tires max psi rating). I typically will run 40-42 in my summer tires when i plan on traveling (hwy) which are 255/35/18 and i can usually see 1 sometimes 2 mpg more hwy (depending on the tire) over the typical 36 i run normally. I typically see 35-45k miles out of them as well (also depending on the tire and treadwear rating) Which brings me to my next point. Not all tires have the same rolling resistance. We could theorize all day long given any tire we choose for an example. Real world statistics/results will greatly vary depending on tire though. In order to accurately say one way or another you would have to pick 1 tire/model and conduct the test with said tire as results of various tires would change greatly.

Mr Marco

Sounds like a class I took at UCDavis. Pr, Professor...is this Professor Maxwell?

http://maxwell.ucdavis.edu/~cole/phy...fluids_ch3.pdf

oo7spy

torque = force * radius

The force is the rolling resistance. When you increase the radius, you increase the amount of torque necessary to overcome the rolling resistance. This is not taking into account a heavier tire, a higher rolling resistance, or any of the other factors. This only is the result of increasing the radius. Now, does the extra power requirement lead to a drop in fuel economy? Just keep the physics in mind.

My vote: YMMV.

oonowindoo

It s simple.

Whenever you increase any weight on your wheels, you will lose mpg. Sometimes it is not very noticeable. of course, there are always exceptions but generally speaking:

You put taller tires = more weight
Wider tires = more weight
Bigger rims in width and height = more weight. (same wheels but different width or height)
Chrome = more weight

jhumbo

iTrader: (1)

Fuzzy, interesting data. Were these the same wheels/rims. Just a bigger size tire on your Explorer?

jhumbo

iTrader: (1)

For the same outer tire diameter then I definitely agree that more weight on the wheels would mean a loss of MPG. I'm just not so sure about a bigger diameter since more ground is covered per revolution.

Effectively, by putting a larger tire on the car, we've changed up the gearing. I'm wondering if the change in gearing is beneficial to highway gas mileage (albeit at the detriment of city gas mileage).

However, from the other replies here it seems that many people seem reasonably confident that the extra weight on the wheels will outweigh any potential benefits from the gearing change made by using a larger tire.

oonowindoo

Unless you are going for superior ground clearance. No one would put thicker tires without changing the wheel size or the tire width.

Wait i have seen one on this board. and he was stupid.

fuzzy02CLS

iTrader: (2)

Yes same stock rims. 15"

On my SRT8 I just got new tires. I was deciding on keeping the stock 255-45-20 or getting 275-40-20's all around. The weight differences on the tires was 3lbs per tire. 12lbs total. That's 12lbs of additional rotational mass. I didn't like that.

jhumbo

iTrader: (1)

But you could put bigger tires and then drop the suspension to achieve the same ground clearance as before the tire change.


In any case, part of the reason I've been thinking about this question is that I had moved from a 18x7.5" rim to an 18x8" rim. When doing that, based on the potential tire size options, the sizes available that were closest to the OEM tire diameter were either 2% larger (45 profile) or 2% smaller (40 profile). I had a tough time deciding whether to go up or down and ended up choosing to go with the smaller size. I since have wondered what differences the larger size might have made. It's just been a curiosity thing, more than anything else. I'm not about to actually go out and put some larger tires on my car hoping to get an extra 5mpgs or anything stupid like that.

fuzzy02CLS

iTrader: (2)

I went from the 235-45-17 on the CLS to 235-40-18. Which is a direct size match. But the rims were a lot lighter then the stock rims so I lost weight there. 29lbs of it. (SSR rims)
In this case I saw my MPG go up.

oonowindoo

Again, no on will put thicker tires and lower it because #1, higher chance of rubbing #2, makes the rims look smaller #3 it will look stupid.

If you had move from 7.5" rim to a 8" wide rim, you can use the same size tires as before. 205 to 225 are all ok with 45 profile. on a 7.5 -8" wide rims.