Weight of Tires/Wheels and MPG Gains

[Quattro Pete]

Originally Posted By: FZ1
You are travelling farther with each revolution,than you know, unless the original tires were 20 or you have had your spedo recalibrated for the 20's

Obviously 20" rims require different size tires than 17" rims. All in all, the overall diameter of 20" rims + tires is about the same as 17" rims + tires so there is no need to recalibrate anything.

 

[RichardSenn]

Originally Posted By: Quattro Pete
Originally Posted By: FZ1
You are travelling farther with each revolution,than you know, unless the original tires were 20 or you have had your spedo recalibrated for the 20's

Obviously 20" rims require different size tires than 17" rims. All in all, the overall diameter of 20" rims + tires is about the same as 17" rims + tires so there is no need to recalibrate anything.



Exactly the tire diameters are 31.6" and 31.9" so they are pretty apples to apples comparisons for how far I'm traveling. Only thing different is the added 20lbs.

 

[RichardSenn]

Just a quick side note to say thanks for all the comments. I've been lurking a while reading then finally joined a month ago and now finally posting.

I'm on several Chevy forums and have not had many replies to any subjects so I find it great that I've received so much input being a new member. Seems like the other forums look at your post count before deciding to respond to a thread.

 

[RichardSenn]

Well, I may do the experiment sooner rather than later. I think I have a buyer for the 20" wheels. I was really hoping to get a unanimous "Yes you will gain 1-2 MPG's with stock" so I would be okay loosing the look of the 20" wheels which I like. But, there seems to be no overall consensus. Even if I can gain 1 MPG better it would be worth it.

 

[Quattro Pete]

I have to ask, do you actually have to do these 28K miles a year in a Tahoe since you already have the Silverado for hauling stuff?

Did you consider trading it in for something smaller and more economical? Or maybe even get another economy beater car that will get you 35 mpg. That's $4320 in savings on gas per year (even more if the gas price goes up), so the beater will pay for itself pretty quickly.

It just seems counter-intuitive to buy a gas guzzler and then spend more trying to make it more economical with questionable results...

 

[kkreit01]

I think you'll gain at least 1 MPG. If you used LRR 17" tires, it would be more. Those Dest ATs have high rolling resistance. I wouldn't use them if your main goal was to gain mpg. Those Michelins aren't very good tires. I would swap to LTX MS2 or Lattitude Tours for the best mpg. Check CR for the lowest rolling resistance.

 

[RichardSenn]

Well, I have the Truck because I spend Sept-January in the woods and the rest of the year going to Archery tournaments, which are most usually not accessible by paved road.

We bought a CX-9 for a good mix, but really the gas mileage on it was not the great. Pretty much on par with my truck so I figured if I'm going to get that kind of mileage I might as well have the vehicle I want and one I could actually repair and work on so I got the Tahoe. Although, I expected the Tahoe to get around 16-17 average based on what a handful (6-7) of people I actually know reported personally getting in there Suburban/Tahoes, hence I started looking to the wheels, which I still believe will get me 2+ mpg's with the switch.

The Tahoe hauls kids, and I had to have a vehicle that 2 Great Danes could fit into. I could be more economical yes, but instead I just choose to cut back else where like not going out to eat a couple nights a week for $40-$50 and taking my own lunch to work saving $5-10 a day.

 

[RichardSenn]

Originally Posted By: RichardSenn
Well, I have the Truck because I spend Sept-January in the woods and the rest of the year going to Archery tournaments, which are most usually not accessible by paved road.

We bought a CX-9 for a good mix, but really the gas mileage on it was not the great. Pretty much on par with my truck so I figured if I'm going to get that kind of mileage I might as well have the vehicle I want and one I could actually repair and work on so I got the Tahoe. Although, I expected the Tahoe to get around 16-17 average based on what a handful (6-7) of people I actually know reported personally getting in there Suburban/Tahoes, hence I started looking to the wheels, which I still believe will get me 2+ mpg's with the switch.

The Tahoe hauls kids, and I had to have a vehicle that 2 Great Danes could fit into. I could be more economical yes, but instead I just choose to cut back else where like not going out to eat a couple nights a week for $40-$50 and taking my own lunch to work saving $5-10 a day.


Actually, let me just say the absolute number 1 reason I bought a Tahoe is because it's big. If could afford for my wife to drive a semi I would do it. Why you ask, because everyday she leaves to take the kiddo to daycare and work I pray that some stupid idiot isn't texting on there phone about to cross the median and hit her or something similar. So, mainly I got it for protection and the price I pay at the pump is worth the peace of mind that maybe that bigger vehicle could save her life in an accident. For that reason alone she will always drive a full size SUV.

 

[OVERKILL]

You know with that kind of mileage, you could have got her an Excursion with the 7.3L, LOL!

 

[RichardSenn]

Originally Posted By: OVERK1LL
You know with that kind of mileage, you could have got her an Excursion with the 7.3L, LOL!


Haha true, but I still hoping to kind of get the best of both worlds. Big vehicle with decent MPG in the 16-17 city 19-20 highway like my truck.

 

[rpn453]

Originally Posted By: Quattro Pete
Originally Posted By: Astro14
IIRC, 1 lb at the wheel is like 10 or more in the mass of the vehicle.

I see this mentioned on various forums, but I have never been able to find math/physics explanation to support this claim. Does anyone have a link?


I don't know where all those crazy numbers come from.

In the case of a wheel, the equivalent mass of a rotating solid disc is 1.5 times its weight. For a thin ring, it's 2.0 times its mass. Those are the two extremes, so it has to be somewhere between that. I originally calculated these values myself but since then I've also seen them published in one of my mechanical engineering textbooks.

I did a quick search and this page seems to have good info:

http://hpwizard.com/rotational-inertia.html

Originally Posted By: HPWizard
Shaving a pound from your tires is equivalent to shaving at most 2 pounds of non-rotating weight. That's PER TIRE, so a pound off each tire could worth close to 8 pounds of weight reduction. For wheels, the multiplier is closer to 1.6, so saving 5 pounds per wheel (20 total) would feel like a static weight reduction of 32 pounds. For brake discs, it can be as low as 1.2.


In the case of this Tahoe, the equivalent mass of the extra 80 lb of wheels is between 120 to 160 lb. For a 5500 lb vehicle, this affects acceleration and braking by about 2.5% and, all else being equal, the extra mass itself increases rolling resistance by about 1.5%. So I'd guess that the total effect of the wheel mass on fuel economy is well under 2%. The wider tire could add a little bit of aerodynamic drag too, but it's almost nothing compared to the drag area of the vehicle and they're not a whole lot wider than those on the smaller rim sizes. The model of tire and level of wear would probably have a bigger impact on fuel economy than anything. If you notice any difference from switching, I'd attribute it primarily to that.

 

[RichardSenn]

Originally Posted By: rpn453
Originally Posted By: Quattro Pete
Originally Posted By: Astro14
IIRC, 1 lb at the wheel is like 10 or more in the mass of the vehicle.

I see this mentioned on various forums, but I have never been able to find math/physics explanation to support this claim. Does anyone have a link?


I don't know where all those crazy numbers come from.

In the case of a wheel, the equivalent mass of a rotating solid disc is 1.5 times its weight. For a thin ring, it's 2.0 times its mass. Those are the two extremes, so it has to be somewhere between that. I originally calculated these values myself but since then I've also seen them published in one of my mechanical engineering textbooks.

I did a quick search and this page seems to have good info:

http://hpwizard.com/rotational-inertia.html

Originally Posted By: HPWizard
Shaving a pound from your tires is equivalent to shaving at most 2 pounds of non-rotating weight. That's PER TIRE, so a pound off each tire could worth close to 8 pounds of weight reduction. For wheels, the multiplier is closer to 1.6, so saving 5 pounds per wheel (20 total) would feel like a static weight reduction of 32 pounds. For brake discs, it can be as low as 1.2.


In the case of this Tahoe, the equivalent mass of the extra 80 lb of wheels is between 120 to 160 lb. For a 5500 lb vehicle, this affects acceleration and braking by about 2.5% and, all else being equal, the extra mass itself increases rolling resistance by about 1.5%. So I'd guess that the total effect of the wheel mass on fuel economy is well under 2%. The wider tire could add a little bit of aerodynamic drag too, but it's almost nothing compared to the drag area of the vehicle and they're not a whole lot wider than those on the smaller rim sizes. The model of tire and level of wear would probably have a bigger impact on fuel economy than anything. If you notice any difference from switching, I'd attribute it primarily to that.


Great info, thanks for putting some real numbers and equations to come up with this. Most everything else is just attributed to opinions or an educated guess.

Well, at this point I'm just waiting for the weekend so I can swap tires and see if all this adds up. I'll be sure to post the results.

 

[rpn453]

Originally Posted By: rshaw125
On the contrary. the extra weight is 'unsprung' weight and would make the vehicle handle and brake worse.


On rough surfaces, I agree. On smooth surfaces, I'd expect the 20" wheels to do better.

 

[rpn453]

Originally Posted By: RichardSenn
Great info, thanks for putting some real numbers and equations to come up with this.


No problem.

I can understand why you want your family in a big vehicle. I'd like to see an effort to make all passenger vehicles more compatible in collisions through weight and bumper height regulations, but that ain't going to happen anytime soon so it makes sense to protect them if you can afford it.

I thought maybe that was the reason that my brother-in-law had my sister driving a Hummer H2 a few years ago. But when that got written off (fallen tree during storm), he gave her a Mini! Her current X5 is probably a good compromise.

 

[CapriRacer]

First, everyone needs to be aware that the rolling resistance of new tires varies wildly. There can be up to a 60% difference in RR between tires of the same size! So comparing tires of different size becomes VERY problematic! If you add different rim diameters to the mix - well, it's almost impossible to sort thngs out.

People also need to be aware that worn tire give better fuel economy than new tires - both because of weight and because of RR.

I am glad rpn453 did the calculations, so I wouldn't have to. Bottomline: You only feel the extra weight when you accellerate. You're losing it when braking, but you are only lossing what you gained during the accel mode.

RR is distance dependent. Travel 10 miles and the fuel lost is twice what it would be if you travled 5 miles.

Aero for wider tires? Yes, but the loss is small.

RR improvements because of bigger tire size? Yes, but it is also small, although it is bigger than aero, so it is better to use wider tires inspite of the aero loss.

Richard, if you wnant to do the experiment, go ahead, but there are so many variables, you're really not going to be able to tell what is affect what!

So if you travel a long distance, you will not feel much of the effect of heavier tires and wheels.

But if you travel in stop and go traffic, the effect will be small!

 

[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!!!

Just recently put LTX MS2 on in 265/70/17 and gave almost all those gains up. Truck is hugely improved in handling, braking, etc., but those big heavy tires must take a lot more power to move!

 

[229]

I spent the money to go to 20's on my Silverado and I would gladly pay $200 shipping to trade you for the smaller wheels. The unsprung weight and decrease in sidewall do affect the ride a small amount. I only had two semesters of physics but I am talking to you from practical experience on the wheel changes. It takes an increase in coefficient of friction to improve traction thereby helping handling and braking but inversely hurting rolling resistance and mpg. I figured you got the vehicle for safety. It's not worth the effort to change the wheels between vehicles. I know, I know....the wheel lugnut torque is 140 lbs/ft!

 

[RichardSenn]

Originally Posted By: CapriRacer

Richard, if you wnant to do the experiment, go ahead, but there are so many variables, you're really not going to be able to tell what is affect what!


Thanks for the info, this site is great!

I know there will be several variables to the experiment but there are some things that will be consistent so I still think it will still be valid.

-The only thing that will change will be the wheels
-The wife will do her best to drive exactly the same (all shifts under 2000 RPM
-Same exact route traveled everyday

I know there will be changes in the weather, traffic, etc.. but it will be a pretty close comparison.

 

[RichardSenn]

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!!!

Just recently put LTX MS2 on in 265/70/17 and gave almost all those gains up. Truck is hugely improved in handling, braking, etc., but those big heavy tires must take a lot more power to move!


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.

 

[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.