New tires

[Valhalla]

For the test to be effective wouldn't it have to be the exact same on everything except rotational mass? If it is a different rubber, at a different PSI made by different manufacturer that designed it slightly different size... I drive the pi$$ out of mine, I'm not surprised by my 16.4 MPG.

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[KreeHBronco]

Driving Line on Youtube just did a pretty decent comparison of Nitto Recon Grappler LT vs regular load rated tires (same size, similar tread, ...) and the results were significant. Part of the issue with higher load tires is the energy needed to deform the tire during rotation (tires at normal operation pressure deform as they roll) which increases rolling resistance which decreased mpg.

 

[Valhalla]

Driving Line on Youtube just did a pretty decent comparison of Nitto Recon Grappler LT vs regular load rated tires (same size, similar tread, ...) and the results were significant. Part of the issue with higher load tires is the energy needed to deform the tire during rotation (tires at normal operation pressure deform as they roll) which increases rolling resistance which decreased mpg.

Inversely, not deforming decreases offroad capabilities. However, thin carcass is easier to puncturable.

 

[crenca]

Driving Line on Youtube just did a pretty decent comparison of Nitto Recon Grappler LT vs regular load rated tires (same size, similar tread, ...) and the results were significant. Part of the issue with higher load tires is the energy needed to deform the tire during rotation (tires at normal operation pressure deform as they roll) which increases rolling resistance which decreased mpg.

Link? I looked here but none of these titles seem to be the video to which you refer:

https://www.youtube.com/@DrivingLine/videos

 

[KreeHBronco]

Link? I looked here but none of these titles seem to be the video to which you refer:

https://www.youtube.com/@DrivingLine/videos

 

[CG32]

Short answer, will you notice the weight? yes at first. Will it be a huge impact and will you care? Probably not after a week of driving on them.

I have gone with larger heavier tires on my last 5 vehicles and after a few drives, it just becomes the new normal. As others have mentioned, the load range and rating will probably be more noticeable for longer and will have more of an impact on the ride quality than weight.

 

[Brian_B]

Wow almost 2MPG difference is pretty significant. Granted they are testing a LR E vs the SL, so that's a pretty big difference. 8 lbs per wheel plus the stiffness.

 

[crenca]

Wow almost 2MPG difference is pretty significant. Granted they are testing a LR E vs the SL, so that's a pretty big difference. 8 lbs per wheel plus the stiffness.

I agree, assuming their methodology is sound...I notice he doesn't say so I am going to assume it's just the dash computer he was reading from one week(s), one tire vs the other, so not reliable probably.

 

[The Green Flash]

On the contrary, it is very significant and will effect suspension response and thus vehicle handling and acceleration (but still a trade off the OP may want to make).

See: difference between sprung and unsprung weight

&

rotational mass

72 pounds is 'very' significant??? Maybe if your background is driving Mazda Miatas.

It's like having a little 10 year old kid in the back seat... or a cooler full of beer and ice. Not sure if I'd even notice it back there...

 

[Valhalla]

72 pounds is 'very' significant??? Maybe if your background is driving Mazda Miatas.

It's like having a little 10 year old kid in the back seat... or a cooler full of beer and ice. Not sure if I'd even notice it back there...

Although I agree with your weight analysis, rotational mass is slightly different. Here is what i have found.

• Magnified Effect: The general rule of thumb is that a reduction in rotational weight (specifically at the wheels/tires, which are also unsprung) feels like a reduction of roughly 2 to 3 times that amount in static, sprung weight. Some sources even suggest a ratio of up to 10:1 for rotational weight at the outer edge of the wheel.
• Handling & Ride Quality: Rotational components like wheels and tires are also part of the unsprung weight (mass not supported by the suspension). Lighter unsprung weight allows the suspension to react more quickly to road imperfections, keeping the tires in contact with the road better, which improves handling, steering response, and ride quality.

So, if you take this as fact; then 72# more rotational weight could be equivalate to 10 children, or 10 coolers of beer. (don't let children drink or drive).

THAT being said take my case of Hutchison wheels and 37 swampers I'm slightly shy of 800# additional simulated weight and 2.5" diameter.
My 2.7 10 speed tuned has never once seemed to notice the weight or size with a 4.7 ratio. I drive semi hard; I get 16.4 MPG 2 door with Winches front and rear and heavy bumpers.

 

[crenca]

72 pounds is 'very' significant??? Maybe if your background is driving Mazda Miatas.

It's like having a little 10 year old kid in the back seat... or a cooler full of beer and ice. Not sure if I'd even notice it back there...

Do some basic research on the terms I listed (start with 'rotational mass') and how they effect vehicle dynamics and acceleration.

Your intuitive approach, i.e. that all mass is equal, is simply wrong.

 

[KreeHBronco]

Although I agree with your weight analysis, rotational mass is slightly different. Here is what i have found.

• Magnified Effect: The general rule of thumb is that a reduction in rotational weight (specifically at the wheels/tires, which are also unsprung) feels like a reduction of roughly 2 to 3 times that amount in static, sprung weight. Some sources even suggest a ratio of up to 10:1 for rotational weight at the outer edge of the wheel.
• Handling & Ride Quality: Rotational components like wheels and tires are also part of the unsprung weight (mass not supported by the suspension). Lighter unsprung weight allows the suspension to react more quickly to road imperfections, keeping the tires in contact with the road better, which improves handling, steering response, and ride quality.

So, if you take this as fact; then 72# more rotational weight could be equivalate to 10 children, or 10 coolers of beer. (don't let children drink or drive).

THAT being said take my case of Hutchison wheels and 37 swampers I'm slightly shy of 800# additional simulated weight and 2.5" diameter.
My 2.7 10 speed tuned has never once seemed to notice the weight or size with a 4.7 ratio. I drive semi hard; I get 16.4 MPG 2 door with Winches front and rear and heavy bumpers.

Good summary!! I think the formula for a rotational inertia has mass a linear term, but the radius is squared. So the size/radius of your tire really has more impact than the mass when it comes rotational effects.

 

[swamp2]

I disagree and the math proves it. Most guys complaining of loss of MPG due to heavier tires do not have the pressure adjusted correctly. Here's why-

Two tires, same diameter (32"), same tread, same size — only difference is weight. One is 50 lbs, the other is 65 lbs. 32" diameter is r = 0.4064 m. Speed is 50 mph, which is 22.352 m/s. Converting weight to mass: 50 lbs = 22.68 kg, 65 lbs = 29.48 kg.
We model the tire as a thin-walled hoop, like a pipe, with the mass concentrated at the tread, instead of a solid cylinder like a bar.
Total kinetic energy per tire at speed v is the sum of translational (moving forward) and rotational (spinning): ½ m v² (forward) + ½ m v² (spinning) = m v²
So at 50 mph-50 lb tire: 11.33 kJ, 65 lb tire: 14.73 kJ
Difference: 3.40 kJ per tire, or 13.6 kJ for all four.
Convert that to fuel use:
13.6 kJ / 0.20 drivetrain efficiency / 120,000 kJ per gallon ≈ 0.00057 gallons

So the extra fuel needed to spin up four tires from 0 to 50 mph is about half a thousandth of a gallon. In real-world terms: nothing. A vehicle making over 300 lb-ft of peak torque barely notices that load, especially compared to the energy required to move a 4000 pound vehicle.

That’s why you don’t lose 2 mpg from 15 lb per corner. The mileage change people see isn’t from weight — it’s from rolling resistance, tread design, and PSI. Think about it this way- if adding 60 or 100 pounds, sprung or unsprung, dropped your mpg by two points, every time you load the family in the truck, you'd drop under 10 mpg. Forget about towing anything.

Very good back of the envelope here. Let's figure out if your hunch on what causes the change in mpg is correct as it's a pretty well known fact that larger tires can and do drop the fuel efficiency of a truck/SUV and the change is often significant, perhaps around 10%, maybe more.

As you stated, tires do loose (to heat) a non-trivial amount of energy from a constantly moving deformation zone, "in to" and "out of" the "stationary" contact patch. This is a function of vehicle speed squared. Below about 50 mph most cars and trucks are loosing very little overall power here to heat, but due to V^2 it is much more significant at 75 mph, maybe about 10 hp total (based on "Fundamentals of Vehicle Dynamics", Gillepsie). This is not the change from one tire to the other, but rather the total loss from all tires. This encompasses all differences in tire design, materials, tread and under/over inflation effects.

Now having a tire significantly underinflated would indeed increase these losses substantially (and beyond 10 hp). That said, I doubt that most folks tracking their mpg and reporting them on web forums are so careless as to having had proper tire pressure before a tire change and then way too low after putting on those nice, fresh, new tires.

But given that this power is continuously being lost/dissipated even a small difference here will add up over time (as opposed to the additional power needed to accelerate a heavier tire with more mass and moment of inertia which only occurs during the acceleration itself - as you covered approximately above).

Let's say the larger, perhaps softer compound and more aggressive tread all make the tire 15% more lossy.

Additional Energy Lost (in say 1 tank ~ 5 h) = .15* 10hp*(1 J/s / .00136 hp) * 5h * (3600 s/h) ≈ 20,000 kJ

(For future reference this difference corresponds to about 1100 W of steady state power)

At 132,000 kJ/gal wink this amounts to about 3/4 gal, which equates to about 0.6 mpg. That's a 4% change in mpg (assuming 15 mpg). Now we are getting somewhere.

I also considered the effect of an increased frontal area on drag and a +2" tire increasing the vehicle height by 1" would probably only add 200W (about 1/4 hp) at 60 mph. This only gets another 1/10th mpg, or now 0.7 mpg.

The extra width of a tire (assume 1.5" wider - indeed may or may not occur) likely contributes about 1% more frontal area and this equates to about an extra 150W (out of ~15kW of air drag at 60 mph). So that's another 10-15% of the additional tire losses.

These together only get me to about a 5% change in mpg (0.8 gal/tank, 300 mi/tank, tank = 20 gal).

A larger tire diameter will cause your engine to run at a different and lower rpm at the same speed. This seems like it would help mpg not hurt it. I'm not too sure how to quantify this, but suspect it is not significant.

Given that the overall efficiency of a Bronco might be closer to 15% (rather than 20%) and the tire design change might also be a 20% more lossy than the factory baseline tire, we could be up to about 1800W loss from the tires (again frictional and aero). This gets us up to about an 8% change in mpg and about 1.6 additional gals/tank. It is my understanding that this is pretty darn ball park for the change from a wider, larger diameter and perhaps more agressive tread (i.e. basically a factory AT to an oversize MT).

Although I could nitpick/improve some details of your calculation, it is fundamentally correct and these additional details validate that the vast majority of the change in effciency when moving to a larger tire, is very likley due to increases in frictional losses associated with shape changes around the contact patch (during steady cruise). There isn't much else on the table, so indeed some process of elimination.

 

[swamp2]

• Magnified Effect: The general rule of thumb is that a reduction in rotational weight (specifically at the wheels/tires, which are also unsprung) feels like a reduction of roughly 2 to 3 times that amount in static, sprung weight. Some sources even suggest a ratio of up to 10:1 for rotational weight at the outer edge of the wheel.

You can see the 2x effect in @Gut s calculation, you get a simple 1/2 mv^2 (energy) from a non rotating mass and an additional 1/2 mv^2 from the energy to get the wheel spinning. His calculation doesn't really show the why but shows the how. The above is a slightly expanded why. So you hear the conclusion that saving 1 lb in a (single) wheel/tire is like saving 8 lb. You get 4x for each of the 4 tires and only a 2x from it being a rotating object. 8x is possible but it's also kind of an exaggeration. It correct only if it is very well defined as I did. 10x - nope, no way to get there... So yes added cargo vs. added wheel or tire weight is indeed fundamentally different. It's more beneficial from a performance (or efficiency) point of view to lose (an equivalent amount of) rotating mass.

This doesn't even touch the added benefits from rotating wheel mass also being unsprung vs. sprung (i.e. moves with the suspension vs. doesn't - like chassis or cargo). There are huge benefits to vehicle dynamics and suspension performance in reducing a vehicles unsprung mass.

*Edited typo of sprung vs. unsprung.

 

[Oldhippie]

Long story short…about everything we do to improve off road performance hurts on road performance…

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