Weight of Tires/Wheels and MPG Gains

[RichardSenn]

Originally Posted By: Quattro Pete
Originally Posted By: RichardSenn
-The only thing that will change will be the wheels

What I think Capri meant is that because you'll be changing several things at once: rim size, tire size, tire brand/model, therefore you will not be able to tell which of these exactly affected MPG and how.

And as he mentioned, RR of the tire itself will have the biggest impact on MPG.

Say you get smaller/lighter rims, but they come with tires that have a higher RR. End result may be that your MPG will actually get worse or stay the same.

Or say that you keep your current rims and just buy new tires that have a lower RR. End result may be that your MPG will improve.



I see what you're saying.

I guess in my mind I see it as the Tahoe setup (275/55/20 Bridgestones) vs. my truck setup (265/70/17 Destination AT's). Because the extra set of wheels I have are the exact same as my truck.

If MPGS do increase then yes I agree the wheel/tire sizes are different so I won't be 100% able to say it was the wheel weight or it was 100% due to the width and rolling resistance of the Bridgestones vs. the Destination AT's

What stays constant:
Wheel Diameter
Tread depth of all tires is 8/32

What changes:
Wheel Weight: From 33lbs to 19lbs
Tire Weight: From 46lbs to 40lbs
Wheels: 20" Chrome to 17" Aluminum
Tire Size: From 275/55/20 to 265/70/17
Tires: From Bridgestone to Firestone

At the end of the experiment all I will be able to say is one setup does better/worse than the other setup.

 

[kkreit01]

Originally Posted By: SteveSRT8
some more anecdotal experience.

2005 Silverado with 265/70/17 stock tire size. Put Michelin 245/70/17 on it at 21000 miles because the OEM Generals were some of the worst tires made!

Picked up almost 2 mpg!!!...

Some of your gains were a result of much smaller diameter tires: 31.6" OE vs. 30.5". Your speedometer was 3.6% too fast, and your odometer was counting more miles per tank.

 

[SteveSRT8]

Originally Posted By: RichardSenn

Did you put the LTX MS2 on both wheels or just the 265?

-Side note, what do you think about the LTX MS2? I priced some yesterday to possibly be the tire to replace my wifes if I get rid of the 20" wheels and go stock and it was $1165 mounted balanced and aligned, which is a lot more than the Destination AT's I put on my truck which I love. But they also have a 70K warranty vs. the 50K warranty with the Destinations.


I have purchased 4 sets so far this year for my fleet trucks. They are by FAR the best of the tires we have ran, at least on trucks and vans. Unreal mileage, seem to last forever! Best rain tire ever, I am thrilled with their performance. It's like you bought a new truck!

 

[OVERKILL]

That pretty much echoes my dad's feelings about them too on the Expedition. He absolutely loves them.

 

[SteveSRT8]

Originally Posted By: kkreit01
Some of your gains were a result of much smaller diameter tires: 31.6" OE vs. 30.5". Your speedometer was 3.6% too fast, and your odometer was counting more miles per tank.


Sorry, this truck has an aftermarket handheld tuner that lets me recalibrate the speedo, all numbers given are corrected for sizing.

 

[RichardSenn]

Originally Posted By: SteveSRT8
Originally Posted By: RichardSenn

Did you put the LTX MS2 on both wheels or just the 265?

-Side note, what do you think about the LTX MS2? I priced some yesterday to possibly be the tire to replace my wifes if I get rid of the 20" wheels and go stock and it was $1165 mounted balanced and aligned, which is a lot more than the Destination AT's I put on my truck which I love. But they also have a 70K warranty vs. the 50K warranty with the Destinations.


I have purchased 4 sets so far this year for my fleet trucks. They are by FAR the best of the tires we have ran, at least on trucks and vans. Unreal mileage, seem to last forever! Best rain tire ever, I am thrilled with their performance. It's like you bought a new truck!


Which one of the MS/2 did you buy? There are two in the E load range (one is GreenX rated and the other is not) and the 3rd is a 113T. I'd probably go the 113T route b/c I'd fear the E rated tires would be to stiff and the ride would suffer.

 

[Astro14]

I am so glad that someone else actually figured this out too...

You know, I started out this thinking after posting the 10 to 1...and to my surprise: 1 KG on the wheel is about equal to 1.5 KG on the vehicle.

So, the physics, more precisely, classical mechanics…been a few years, but here is my rough derivation:

For linear acceleration: KE = 1/2 mass x velocity squared

For rotational acceleration: KE = 1/2 I w squared, where w = angular velocity (Omega) and I = Moment of rotational inertia

(note, I am using I=m x r squared / 2 – i.e. I am treating the additional KG in the tire/wheel as a thin hoop of mass distributed at the edge, which would be only a slight overestimation, since in low profile tires, that is where the mass is concentrated...)

Take 1 KG of mass, accelerate to 65 MPH.

65 MPH = 29 M/sec

KE then for 1 KG in cargo/vehicle is 1/2x1x29 squared = 420 joules

For the 1 KG of tire/wheel mass, it gets a bit more complex.

In the case of the tires on this truck:
• r = .40 M (diameter of the tires was roughly 800 MM).
• I = (1x.4 squared/2) = .08
• W (at 29 M/sec) = 72.5 (Omega is in radians/sec, or about 23 rotations/sec)
So, KE (rotation) = 1/2 .08 times 72.5 squared = 210 joules.

But, that mass is also accelerated linearly as well, so you have to add its linear KE of 420 joules…for a total of 630 joules...Made simple, I come up with 1KG on the tire being worth 1.5 KG on the vehicle in terms of increased KE…

The real question in this calculation is: did I think of all the factors? I mean, I think so...

 

[SteveSRT8]

Originally Posted By: RichardSenn

Which one of the MS/2 did you buy? There are two in the E load range (one is GreenX rated and the other is not) and the 3rd is a 113T. I'd probably go the 113T route b/c I'd fear the E rated tires would be to stiff and the ride would suffer.


Sorry man, the tires are all E rated as these vans weigh in at around 9200 pounds as they leave in the morning! The tires still dramatically improve the ride even with 80 cold psi in them.

The ones on the Silverados are standard load and we are at 38 front and 34 rear cold psi.

 

[rpn453]

Originally Posted By: Astro14
The real question in this calculation is: did I think of all the factors? I mean, I think so...


It looks good to me except that the formula you used to calculate the inertia is that of a solid disc, which is why you ended up with a factor of 1.5 instead of 2. I = m x r^2 for a thin ring, but if you rotate it about either of the other central axes the value is half of that. So you may have just grabbed that equation instead.

It's good to have this recorded on BITOG so I don't have to search too far or redo the calculation if I ever need this info again!

 

[crosseyedwx]

Just to throw in a different approach... I'd switch over to 17's for the simple fact that tires would probably be a little cheaper come time for replacement, and will probably last longer than the 20's.

 

[RichardSenn]

Originally Posted By: crosseyedwx
Just to throw in a different approach... I'd switch over to 17's for the simple fact that tires would probably be a little cheaper come time for replacement, and will probably last longer than the 20's.


Good point. I've thought about this as well.

Pretty sure the 17's are going to go on the Tahoe regardless. I drove it again last night and you can really feel every little bump in the road with those 20's, the 17's ought to be a much smoother ride.

She's only got a half a tank of gas to get through before I can try my little experiment.

 

[Astro14]

Originally Posted By: rpn453
Originally Posted By: Astro14
The real question in this calculation is: did I think of all the factors? I mean, I think so...


It looks good to me except that the formula you used to calculate the inertia is that of a solid disc, which is why you ended up with a factor of 1.5 instead of 2. I = m x r^2 for a thin ring, but if you rotate it about either of the other central axes the value is half of that. So you may have just grabbed that equation instead.

It's good to have this recorded on BITOG so I don't have to search too far or redo the calculation if I ever need this info again!


Ah...thanks for catching that. So, if I had used the formula for a thin annulus, I would have come up with 2:1 since I = m x r squared.

Still, 2:1 is a dramatic difference from the 10:1 I have seen published/talked about elsewhere...which is why I was hoping that someone else would validate my thinking...

If it were my truck, I would go with 17s, just to save the $$ on future tires...I have driven a few Escalades (thank you Hertz) and with 20s on them , I found the ride was poor...which surprised me, since the Yukon (again, thank you Hertz) is a great riding truck. With that and the increased cost of 20s...who wants them?

 

[OtisBlkR1]

i went from stock factory 20's on my silverado with factory p rated tires, to Firestone destinations in the factory size on my factory 20's and lost 2mpg, i then downsized rim to a 18 (but went from factory 8.5 to a 9 inch rim width ) and a much wider 325 tire and TOTALLY destroyed my MPG, next week im going back closer to factory width on those 18 inch rims with 275 tires, im very intrested in seeing what my mpg does.. i went from 19-21 highway to 16-17hwy and right now the computer is showing 14mpg mixed driveing.. this is with the 325/60/18's on it.. i have another thread about the Cooper AT/3 I ordered up on here. i can tell you tire weight, and width deffinately played a roll on my truck.

 

[RichardSenn]

Okay, well we've gone through 2 tanks of gas now on the Tahoe and tonight I swapped out the wheels with my Silverado and topped off both fuel tanks from the same station and pump.

I can tell the braking has drastically improved on the Tahoe with the lighter 17" wheels and it seems to have a little more get up and go to it. Didn't notice to much of a difference with my truck and the 20's except for braking was a little less responsive but not much. Off course all of this is just my "feel" of the vehicles so I'm interested to know what the MPG comes out to in the end.



Last 2 fill-ups on the Tahoe:
317 Miles/22.9 Gallons = 13.84 MPG
327 Miles/22.7 Gallons = 14.41 MPG

Last 2 fill-ups with Silverado:
378 Miles/21.4 Gallons = 17.6 MPG
401 Miles/22.3 Gallons = 17.9 MPG

Sorry for the pic quality. I didn't feel like getting the SLR out and decided my cell phone would suffice.

 

[OtisBlkR1]

the wheels look better on your truck... i deffinately noticed my truck rode better with the smaller rims and more rubber, 20's on trucks look trick, but those skinny sidewalled tires just killed my ride..

 

[spasm3]

The factory wheels just look better.

 

[RichardSenn]

Well, I filled the Tahoe up today after the wife drove it all week on the stock tire/wheel set up. I already knew the MPG would be higher due to Driver Information Center so I purchased new tires yesterday.

Tahoe:
371 Miles/23.1 Gallons = 16 MPG

So, all I really know is the stock set up does better than the 20's and I think the wheel weight plays a huge role in MPG.

Not sure what the truck is going to get, I'll probably fill it up tomorrow after work.