New tires

[crenca]

This doesn't even touch the added benefits from rotating wheel mass also being sprung vs. unsprung (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 sprung mass.

That said, occasionally engineers intentionally add more unsprung weight to achieve certain suspension/vehicle dynamics. In the sixties (maybe it was the fifty's) Jaguar moved their rear brake rotars inboard, close to the center diff on the rear axle, to reduce unsprung mass. They kept this innovation on the XJ(6, usually) up until this century when they moved it back outboard because (at least they claimed) they wanted more unsprung mass...of course it could have been mere cost savings but they maintained it was for dynamic qualities.

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[Gut]

... 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.

...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.

...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.

My post was solely in response to those that claim a mileage loss due to increased weight. There are many other factors that have a larger influence on mileage, including tire size, tread design and pressures.

I disagree with your statement about pressures. Most people do not adjust the tire's pressure for the weight of the truck, they either run it at the pressure on the door or the pressure on the sidewall. The door states the correct pressure for that brand and type of tire. Any change from that needs an adjustment. Event the same brand and size may need different than stated pressures due to different construction like a change in load rating. Optimum wear would see the entire width of the tread making even contact. Optimum MPG needs a crown for less rolling resistance.

Lower RPM doesn't necessarily mean better mileage since at a given RPM, a heavier load requires more fuel to maintain that RPM.

 

[crenca]

Lower RPM doesn't necessarily mean better mileage since at a given RPM, a heavier load requires more fuel to maintain that RPM.

It has been a while since I went down this rabit hole, but the determination is hard to get at as there are several factors (e.g. efficiency at any give torque output at any given rpm, etc.). That said it is more often then not better to be at a lower rpm even if the throttle is (realitive to a higher rpm/lower gear) is more open...

 

[swamp2]

I disagree with your statement about pressures. Most people do not adjust the tire's pressure for the weight of the truck, they either run it at the pressure on the door or the pressure on the sidewall. The door states the correct pressure for that brand and type of tire. Any change from that needs an adjustment. Event the same brand and size may need different than stated pressures due to different construction like a change in load rating. Optimum wear would see the entire width of the tread making even contact. Optimum MPG needs a crown for less rolling resistance.

So you think most people have improperly inflated tires? I disagree, I think most folks are between manufacturer spec and an even (optimal) wear pressure. Folks end up under inflated from the door jamb label both on purpose (when they know a bit) or due to just not checking often enough and a natural very, very slow loss of pressure. Unless people are trailering, a load adjustment isnt really necessary.

So you said prior:

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.

That's not quite right as it does not logically follow that because the increased mass of an alternate tire alone doesn't contribute much to a worse mpg, that the cause of a larger tire exhibiting a lower mpg is under inflation. On the contrary, my post shows that one of the only reasonable causes of lower mpg is tire losses, but tire losses certainly are not solely from under inflation. I suspect, but certainly can't prove, that tire losses are more about design, tread, compound and size differences rather than under inflation. In other words, larger (and often wider and perhaps more aggressive) tires simply have more frictional losses mostly in the tire itself near the contact patch, and perhaps also from rolling resistance.

Also, unfortunately, the door jamb absolutely does not state the "correct" pressure for the tire and vehicle. There really isn't a single "correct " value. It states a pressure that crowns the tire and sacrifices both wear and performance in exchange for improved mpg (of course all for the sake of CAFE standards). It sounds like you know this, but it's not what you wrote above.

 

[swamp2]

That said, occasionally engineers intentionally add more unsprung weight to achieve certain suspension/vehicle dynamics. In the sixties (maybe it was the fifty's) Jaguar moved their rear brake rotars inboard, close to the center diff on the rear axle, to reduce unsprung mass. They kept this innovation on the XJ(6, usually) up until this century when they moved it back outboard because (at least they claimed) they wanted more unsprung mass...of course it could have been mere cost savings but they maintained it was for dynamic qualities.

Hadn't heard this story. Very odd.

I seem to have also typo-ed sprung and unsprung mass prior. Unsprung is the stuff that moves with the suspension or the suspension itself.

I'm going to actually correct my post.

 

[Gut]

So you think most people have improperly inflated tires?

Absolutely.

I don't know if there's any way to prove it empirically but anecdotally, my years of a mechanic and parts guy will attest that most people don't even know how to put air in their tires let alone read a tire gauge.

Most reputable sources, and the Internet, cite two major reasons for premature or unusual tire wear. Alignment issues and improper inflation.

 

[B-Dog15]

Absolutely.

I don't know if there's any way to prove it empirically but anecdotally, my years of a mechanic and parts guy will attest that most people don't even know how to put air in their tires let alone read a tire gauge.

Most reputable sources, and the Internet, cite two major reasons for premature or unusual tire wear. Alignment issues and improper inflation.

I cannot confirm or deny that most people have under inflated tires... HOWEVER it is shocking the amount of people who don't even know that you can put air into your tires. I once had a co-worker who didn't know you could put air in your tires at all... When I was explaining this to him he was in absolute shock. Mind you this was a 26 year old man.

 

[userdude]

I cannot confirm or deny that most people have under inflated tires... HOWEVER it is shocking the amount of people who don't even know that you can put air into your tires. I once had a co-worker who didn't know you could put air in your tires at all... When I was explaining this to him he was in absolute shock. Mind you this was a 26 year old man.

How long before he figured out this wouldn't work??

 

[RobWTX]

I took math in High School, back in the 80's. Spent about 30-45 minutes, and figured I should share this with the group. Here is the one thing you seem to be forgetting.


don’t forget the official Door Coefficient, which obviously impacts aerodynamic laminar flow around Broncos:

Meaning the tire size barely matters — what REALLY determines MPG is whether you chose the correct number of doors.

 

[Valhalla]

I took math in High School, back in the 80's. Spent about 30-45 minutes, and figured I should share this with the group. Here is the one thing you seem to be forgetting.


don’t forget the official Door Coefficient, which obviously impacts aerodynamic laminar flow around Broncos:

Meaning the tire size barely matters — what REALLY determines MPG is whether you chose the correct number of doors.

Although I agree some people need moar.

 

[HertiageMerica]

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.

Could just look at the area moment of inertia for the stock vs new tire and calculate the ratio since everything else will drop out the spinning components need to have the alpha to the math to math right but it really is still negligible in the end.

 

[DALOLA]

I cannot confirm or deny that most people have under inflated tires... HOWEVER it is shocking the amount of people who don't even know that you can put air into your tires. I once had a co-worker who didn't know you could put air in your tires at all... When I was explaining this to him he was in absolute shock. Mind you this was a 26 year old man.

I would've pulled his card on the spot.

 

[B-Dog15]

I would've pulled his card on the spot.

I struggle calling him a man when I tell that story…

 

[DALOLA]

I struggle calling him a man when I tell that story…

It's part of the problem these days, too few are teaching young men how to be men.

 

[Valhalla]

No rite of passage.

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