Tire/MPG Question

[4RunnerAU]

Hey all - I have a 2015 SR5. I upgraded my stock wheels/tires to stock size (265/70r17) Hankook Dynapro ATMs on MB TKOs. Looking at the weight specs on stock wheels and tires vs my upgrades, I shaved off ~6lbs/tire. I was expecting my MPGs to stay the same since my wheel/tire weight drop would offset the aggressive tread and tire size remaining constant...but I have dropped about 0.5mpg since switching (driving conditions staying constant).

Can I attribute this loss in MPGs solely to the aggressive tread on the new tires?

(Not that it matters, just curious).

 

[Swarty]

Too many variables...But probably heavier tires

Can I attribute this loss in MPGs solely to the aggressive tread on the new tires?

(Not that it matters, just curious).

So many things affect mileage... Temp of both the air and the street, winter vs summer gas, humidity, wind speed, carrying more stuff, Our sense of "driving the same" is probably not all that accurate, etc. So many things.

But assuming all things equal:
Weight and tread matters but where the weight is located matters more.
If you have heavier tire on a lighter rim you may have a lighter wheel combo but with a higher rotational inertia. You can have less weight overall but more weight at the outer edge of the tire where it takes more torque to accelerate. If it helps, think of a hammer: The hammer never changes weight but it takes more force to swing it correctly than it does to hold the head and swing the handle. Rotational inertia goes up as the weight moves away from the center.

The tire weight matters more for mileage than the rim weight.

 

[4RunnerAU]

Thank you for the response - I am not "physics-ally" inclined so to speak.

 

[T4RDriver]

I was surprised to find that new TRD wheels + load range 'C' BFG KO2's in stock size cost me ~1.3 MPG. The KO2's are ~6 lbs/tire heavier than the stock Dunlops. Not sure about the TRD wheel weight differential. In the end, I don't really care about the MPG hit since I gained the performance benefits of having tires better suited to normal 4Runner tasks.

 

[spooln30]

Did you actually weight the stock wheel/tires and the aftermarket ones?

 

[4Surfin]

what is the actuall tire size compared to stock?

 

[03_4x4Runner]

Here is the answer. Tires and Fuel Economy - Four Wheeler Magazine

 

[4RunnerAU]

Did you actually weight the stock wheel/tires and the aftermarket ones?

No - just used specs from DTD. I guess i could look at shipping weight.

 

[4RunnerAU]

what is the actuall tire size compared to stock?

Will look - forgot to measure the stock tires.

 

[wfo9]

The weight will have more to do with acceleration... although it does impact mpg also. The tread pattern/width.. will have greater impact on mpg at hwy speed. Tread pattern and compound is a huge factor overall.. So yes... this is why you lost mpg.

 

[spooln30]

I've had ordered a set of tires for my Supra and when I got them I weighed them. They were actually 2 lbs heavier per tire then what they were listed on tirerack.

 

[4RunnerAU]

The weight will have more to do with acceleration... although it does impact mpg also. The tread pattern/width.. will have greater impact on mpg at hwy speed. Tread pattern and compound is a huge factor overall.. So yes... this is why you lost mpg.

On my last truck, a Jeep Wrangler JK, I put 33.5" Duratracs on 15" wheels and lost 0MPG. They replaced 225/75r16 stock tires. I got 21MPG on the hwy. So, tread pattern is important but it isn't everything.

I honestly think that the 2nd poster got it right, but I wonder if wheel diameter plays a big part in it. I guess that a smaller diameter wheel takes less energy to spin the wheel, right?

I am keeping an eye on it. My MPGs went up a little bit.

 

[Hellamatic]

The tires probably aren't the same size. There are a lot of variations among manufacturers for a tire that is the "same" size. If the new ones are a tad bigger, not only will they VERY slightly decrease gas mileage, but they will also very slightly throw off the odometer enough to possibly skew the calculation. They'll be likely very close to the same size as the other tires, but the horseshoes/hand grenades rule does apply to some extent.

As someone else said, so many factors can affect mileage - wind, inflation pressure, (argueably) the brand of fuel you use, number of lights you stop at, mood, moon phases, planetary alignment. Don't panic yet. Drive for a bit and see what happens.

 

[DesertCanyons]

Accurate gas mileage

The tires probably aren't the same size. There are a lot of variations among manufacturers for a tire that is the "same" size. If the new ones are a tad bigger, not only will they VERY slightly decrease gas mileage, but they will also very slightly throw off the odometer enough to possibly skew the calculation. They'll be likely very close to the same size as the other tires, but the horseshoes/hand grenades rule does apply to some extent.

As someone else said, so many factors can affect mileage - ...

You've hit on a point that most of us haven't considered, me included. The odometer is off with the tire size change. The odometer only reads tire rotations and updates the odometer based on a fixed tire size. It does not measure distance travelled directly. Look at the tire fitment guide thread by 1Engineer for an approximation of the difference tire size can make.

If you're reading your fuel economy from the vehicles calculation shown on the dash, you're probably not getting accurate numbers, even in the best case with new stock tires.

To compensate it may be necessary to measure the tire circumference directly and adjust the mileage reading according to the ratio of the circumference of your tires to stock tires. That is, to find the correct mileage divide mileage shown by the circumference of the stock tire and multiply by the circumference of your tire (increasing the actual mileage driven). We should also be dividing total real miles travelled by the volume of gas used, not relying on the readout from the vehicle.

Start with a full tank and calculate after using several tank fulls to get a more accurate reading and reduce the error induced by variation in the fill volume of the first and last tank fill.

I'm guessing that most people here know how to measure gas mileage, but there it is in all it's mundane depth. layball:

 

[Hellamatic]

You've hit on a point that most of us haven't considered, me included. The odometer is off with the tire size change. The odometer only reads tire rotations and updates the odometer based on a fixed tire size. It does not measure distance travelled directly. Look at the tire fitment guide thread by 1Engineer for an approximation of the difference tire size can make.

If you're reading your fuel economy from the vehicles calculation shown on the dash, you're probably not getting accurate numbers, even in the best case with new stock tires.

To compensate it may be necessary to measure the tire circumference directly and adjust the mileage reading according to the ratio of the circumference of your tires to stock tires. That is, to find the correct mileage divide mileage shown by the circumference of the stock tire and multiply by the circumference of your tire (increasing the actual mileage driven). We should also be dividing total real miles travelled by the volume of gas used, not relying on the readout from the vehicle.

Start with a full tank and calculate after using several tank fulls to get a more accurate reading and reduce the error induced by variation in the fill volume of the first and last tank fill.

I'm guessing that most people here know how to measure gas mileage, but there it is in all it's mundane depth. layball:

To take it a step farther, you can use a gps as an odometer for figuring fuel mileage. But at the end of the day, are you REALLY curious enough to take it to the nth degree? I know my mileage display is skewed, but I just use it as a rough comparative gauge - ie, "I got better mileage on this trip, shitty mileage on this leg, blah blah" - mostly I just enjoy rolling around in my 'Runner!

 

[1engineer]

So many things affect mileage... Temp of both the air and the street, winter vs summer gas, humidity, wind speed, carrying more stuff, Our sense of "driving the same" is probably not all that accurate, etc. So many things.

But assuming all things equal:
Weight and tread matters but where the weight is located matters more.
If you have heavier tire on a lighter rim you may have a lighter wheel combo but with a higher rotational inertia. You can have less weight overall but more weight at the outer edge of the tire where it takes more torque to accelerate. If it helps, think of a hammer: The hammer never changes weight but it takes more force to swing it correctly than it does to hold the head and swing the handle. Rotational inertia goes up as the weight moves away from the center.

The tire weight matters more for mileage than the rim weight.

Exactly. This condition is exacerbated if you are driving in a city. If you do most of your miles on the interstate then not so much. In stop and go driving, you have to accelerate the weight of the tire up to speed many times. That is where you use a lot of energy and reduce efficiency.

 

[Jetboy]

Generally the weight of the tire doesn't affect mpg much. I doubt 5lbs per tire either way is measurable in terms of mpg. It's roughly equal in terms of power required to accelerate of a 2:1 ratio with non-rotational mass. Maybe a lb at the tread could have closer to a 3:1 factor, but either way it's not much. So 5lbs per tire is roughly the same as dropping 5*4*2 = 40lbs of load. I don't think 40lbs difference would be measurable in MPG difference, nor do I think 5lbs per tire is noticeable. I don't measure lower mpg with a passenger that makes more difference than the tire weight would make - now if you slap on a set of 16R20 XZLs, yeah, 200lb tires will be a big difference.

What really makes the difference in mpg is rolling resistance. More aggressive tread tends to have larger blocks that take more energy to flex. And more off-road oriented tires tend to have thicker sidewalls and tread rubber that also takes more energy to flex. The increases rolling resistance is probably a lot more important than the weight.

 

[Swarty]

Sort of...the Hankook ATM is a pretty smooth roller.

Generally the weight of the tire doesn't affect mpg much. I doubt 5lbs per tire either way is measurable in terms of mpg. It's roughly equal in terms of power required to accelerate of a 2:1 ratio with non-rotational mass. Maybe a lb at the tread could have closer to a 3:1 factor, but either way it's not much. So 5lbs per tire is roughly the same as dropping 5*4*2 = 40lbs of load. I don't think 40lbs difference would be measurable in MPG difference, nor do I think 5lbs per tire is noticeable. I don't measure lower mpg with a passenger that makes more difference than the tire weight would make - now if you slap on a set of 16R20 XZLs, yeah, 200lb tires will be a big difference.

What really makes the difference in mpg is rolling resistance. More aggressive tread tends to have larger blocks that take more energy to flex. And more off-road oriented tires tend to have thicker sidewalls and tread rubber that also takes more energy to flex. The increases rolling resistance is probably a lot more important than the weight.

That is true with very aggressive tread.
Total inertia from tires is straight line inertia + rotational inertia ( which is a product of a square and not a linear increase by distance). Weight on the outside edge of a 32" tire should have an almost 8X affect vs weight in the vehicle. Even keeping the tire weight even but shifting the weight from the rim to the tread is about a 35% change in position which would be a 82% increase in rotational inertia for that portion of the mass.

 

[Jetboy]

I'm not sure I'm following your math. Maybe I'm wrong - if so help me sort out where I've gone wrong.


m = total mass
v = velocity
I = moment of inertia
w = rotational velocity = (1/r)(v)
r = tire radius (16")


The effective kinetic energy of wheel is the sum of 1/2mv^2 + 1/2I(1/r)^2(v)^2

To back out effective mass - how much it feels like it weighs - we can come up with the formula: Mass(effective) = m(actual) + I(1/r)^2

Then we can make the ratio of M(actual)/M(effective) and have a pretty good comparison.

For every 1lb at 16" from the hub (on the tread surface), you have an effective mass of 1 + 0.37459(1/.4)^2 = 2.34lbs. So by my calculation every lb you add at the surface (the maximum effect) would be equal to adding 2.34lbs of weight on the frame.

I'm not sure that the rotational inertia is the problem with mpg. Especially considering it's only having an effect for may be 30% of the driving time. I think the real culprit is rolling resistance. And intuitively this also makes sense becasue the mpg usually is lower with certain tires in both city and highway.

 

[4RunnerAU]

Thanks to everyone for not treating this like "just another MPG" thread. Learned a ton.