Weight of Tires/Wheels and MPG Gains

[RichardSenn]

I bought the wife a used 2006 Tahoe with stock 20" chrome wheels and 275/55/20 Bridgestone Duelers. After the first tank of gas and only averaging 14 mpg with pure gas (no 10% ethanol) I thought something was wrong so I got a wild hair and figured it must be the 20" tires/wheels bringing my MPG's down because they weigh so dang much.

Turned out I was right. Below are the results of weighing 3 stock wheels/tire setups and the current setup on the Tahoe weighs 20lbs extra over the stock setup per corner. I've got an extra 80lbs of rotational mass just due the wheel and tire setup.

Any guess as to how many MPG's I can pick up dropping 20lbs per corner and going back to stock?

Original Setup Wheel 1 (17" Aluminum from 2006 Chevy Silverado with 265/70/17 Firestone Dest. AT's) - 19lbs (Total wheel/tire weight was 59lbs and the tire weighs 40lbs)
Wheel 2 (17" Aluminum from 2006 Chevy Tahoe with 265/70/17 Michelin LTX A/S E Load) - 18lbs (Total wheel/tire weight was 67lbs and the tire weighs 49lbs)
Current Setup Wheel 3 (20" Chrome from 2007 Chevy Tahoe with 275/55/17 Bridgestone Dueler AT RH-S) - 33lbs (Total wheel/tire weight was 79lbs and the tire weighs 46lbs)

 

[eljefino]

Maybe .2 mpg in both city/highway. I bet the 20s stick out in the air causing wind resistance too.

Are you looking for stock take-offs?

 

[Quattro Pete]

I'd guess your MPG will go from 14 to 14.5 or 15.

Welcome to BITOG!

 

[229]

That is the mileage they get. Don't take the "cool" and safety out for 0.2 mpg. The 20" option would cost around two grand if you wanted it. They add a lot of handling and braking.

 

[A_Harman]

If you're doing a lot of stop & go driving, the heavier tires and wheels will have a pretty significant effect on fuel economy. It's not only that fact that the wheel assemblies weigh more, but the fact that they rotate, which multiplies the inertia effect by a factor of pi.(3.14, you know.) It takes much more energy to accelerate and decelerate the larger-diameter wheels. By adding 80 pounds of rotational weight, you've basically added 250 pounds of inertia to the car. This is why race car builders are always striving to reduce rotational mass.

You wouldn't notice as much an effect on fuel economy if most of your driving is highway, where you accelerate up to a steady cruising speed and hold it for a long period of time. One small effect would be if the 20" tires are wider, which would add to the frontal area and increase aero drag.

 

[RichardSenn]

I'm betting it would get at least 2mpg better, not 0.2 better. At least that's what I'm hoping.

Stock tires would be 8.1" for tread width and the current tires are 9.5" for tread width.

I wish there was a way to know for sure, like said above I'd hate to loose the look but even if I got 2mpg better city/highway it would be worth it in the long run. Especially since I can sale the current setup and not be out any $$.

For example my Chevy CC with the same engine, tranny, gears, etc. gets 17.5 average and about 19 on the highway. I know the Tahoe is bigger but I was hoping for around 16 or so. That 14MPG I posted is about 95% highway.

 

[Scum_Frog]

Stock Tire - 275/55R20
Search Tire - 265/70R17

Section Width: 10.82 in 275 mm
Section Width: 10.43 in 265 mm

Rim Diameter: 20 in 508 mm
Rim Diameter: 17 in 431.8 mm

Rim Width Range: 7.5 - 9.5 in
Rim Width Range: 7 - 9 in

Overall Diameter: 31.90 in 810.26 mm
Overall Diameter: 31.60 in 802.64 mm

Sidewall Height: 5.95 in 151.13 mm
Sidewall Height: 7.30 in 185.42 mm

Radius: 15.95 in 405.13 mm
Radius: 15.80 in 401.32 mm

Circumference: 100.2 in 2545.0 mm
Circumference: 99.27 in 2521.4 mm

Revs per Mile: 652.0
Revs per Mile: 658.2

Actual Speed: 60 mph 100 km/h
Speedometer1: 60.5 mph 100. km/h

Speedometer Difference20) -
Speedometer Difference: (17) 0.950% too fast

Diameter Difference20) -
Diameter Difference: (17) 0.95%


But your actual Diameter, Circumference, Revs per Mile and Radius are so similar between those two tires ( as they should be for proper fitment) I would figure very little difference between the two.

http://www.1010tires.com/tiresizecalculator.asp

 

[RichardSenn]

That's just it, everything is so close except for the the 20" tires are an inch wider and weigh 20lbs more. I'm just really stuck on the fact that the extra 20lbs has to whats hurting the MPG's.

I guess, my next experiment will be to swap the tires on my wife's Tahoe (20") with the tires on my Silverado (17"). If she sees an MPG gains and I see MPG loses we'll know. I'll know pretty quick considering my route never changes and out of the last 10 fill ups (I log it) I pulled up to the pump with exactly the same miles on the trip meter 7 of those 10 times.

I'm hoping this second tank of gas on the Tahoe is better. We may have been a little heavy on the gas since we just got it and it was new to us, and I did change out the oil to synthetic Mobil 1 5W-30 with a Mobil 1 filter.

 

[rshaw125]

Originally Posted By: 229
That is the mileage they get. Don't take the "cool" and safety out for 0.2 mpg. The 20" option would cost around two grand if you wanted it. They add a lot of handling and braking.


On the contrary. the extra weight is 'unsprung' weight and would make the vehicle handle and brake worse.

 

[Hokiefyd]

Yes, the extra weight is hurting handling and braking, not helping it. I'm all for reducing unsprung mass, and think you're doing the right thing, Richard. After you make the switch, let us know what your results are!

 

[Quattro Pete]

Originally Posted By: RichardSenn
I'm just really stuck on the fact that the extra 20lbs has to whats hurting the MPG's.

But you have to consider your point of reference. On a small car with a small engine, a difference of 20 lbs per corner (unsprung) would indeed be significant. On a 5000+ lbs Tahoe with a big V8, it would be less significant. It's kind of like with the use of A/C: on a car with a small weak engine, the use of A/C affects MPG much more than on a larger engine.

To go from 14 mpg to 16 mpg, that's more than 14% improvement in MPG. I'd say it's somewhat unrealistic to achieve it with just lighter wheels/tires. You'll be lucky to see half that.

But in any case, it's worth a try to go with a lighter setup. Keep in mind the rubber compounds in specific tires (LRR) can affect MPG as well. So can tire pressures. However, the biggest MPG gains are to be had from modifying the behavior of the nut behind the wheel.

 

[bepperb]

Assuming this is a GM option (and not a dealer option) EPA sticker has to reflect those tires. But the EPA sticker probably doesn't even change (between option packages) since the difference isn't going to be anywhere near 2mpg.

Best of luck, and I'd love to hear some results, but I'd bet 1mpg tops, and I'm doubtful of even that.

 

[FirstNissan]

My guess is your mpg change is going to be nill. I had an old explorer during college, I did everythign to lighten the car to increase its pathetic mpg's. Pulling out the spare, rear seats, smaller wheels, removing roof rack, I got a 1 mpg difference.

 

[JHZR2]

Not much at all. You have a big vehicle with a big engine and likely a wife with a heavy foot who doesnt respect the physics of the situation.

 

[IndyIan]

It would be interesting to hear from Capriracer if all else being equal, how do shorter or taller side walls on tires effect rolling resistance?
Also I think getting new tires is a chance to adjust gearing for mileage. If you do alot of highway driving and the truck has no trouble staying in top gear with the TC locked you could probably get some taller tires to drop cruising rpms more.
If its used in the city alot you might want to get shorter tires to allow the truck to get into top gear more often with the TC locked.
Overall the tires aren't going to make a huge difference but with some LRR tires and some driver mods getting 10 or 20% better isn't out of the question.

 

[MNgopher]

You'll net a MPG increase, but not more than a few tenths of a MPG.

Stepping back though, all of this is based on one fill up? I'd wait a bit before I passed judgement on the fuel mileage. My new to me F150 got terrible fuel milege on the first tank, but it improved steadily thereafter after the truck and I got acquanited with each others driving style...

Although I would say 14 mpg for a full sive SUV isn't crazy depending on driving style and type. May even be what you can expect.

 

[Astro14]

It's not just a question of unsprung weight.

Those 20s have a much greater rotational moment of inertia (angular mass) than the 17s. It takes far more power (from the engine = gasoline burning) to get them rotating than it does the 17s. All that energy is lost to heat when you brake. Granted, the OP says mostly highway, but there are still small acceleration/decelerations happening there. In general, much heavier wheel/tire combos have a dramatic effect on MPG, not because of their weight, but because of the energy used to accelerate them in rotation. IIRC, 1 lb at the wheel is like 10 or more in the mass of the vehicle.

So, 80 lbs more at the wheels could have the effect of several hundred lbs of cargo...and that will make a difference in your MPG.

Would love to see the results of your wheel swap...

 

[RichardSenn]

Originally Posted By: Astro14
It's not just a question of unsprung weight.

Those 20s have a much greater rotational moment of inertia (angular mass) than the 17s. It takes far more power (from the engine = gasoline burning) to get them rotating than it does the 17s. All that energy is lost to heat when you brake. Granted, the OP says mostly highway, but there are still small acceleration/decelerations happening there. In general, much heavier wheel/tire combos have a dramatic effect on MPG, not because of their weight, but because of the energy used to accelerate them in rotation. IIRC, 1 lb at the wheel is like 10 or more in the mass of the vehicle.

So, 80 lbs more at the wheels could have the effect of several hundred lbs of cargo...and that will make a difference in your MPG.

Would love to see the results of your wheel swap...


This was my thoughts exactly.

I know it was just one tank but this next tank will for sure be the indicator. We both drive the same 25 mile route to work and 25 miles back home at the same time of day every day so the route is consistant. I know the driving habits are consistent because the wife always watches the RPMS and all the shift are at or below 2000 RPM. In fact it's funny she finds it challenging to not accelerate off the line and is the first to mention it to me when I shift above 2,000 RPMS.

I'll see how this tank does, and pure curiosity will probably cause me to swap the tires out for a comparison next week.

 

[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?

By the way, I agree that unsprung weight and rotational mass is not the same as stationary mass, but I'm curious about this 10x claim.

 

[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