Gains with Thin Tires? Lighter wheels? - Fuelly Forums

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Old 04-06-2007, 01:35 PM   #1
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Gains with Thin Tires? Lighter wheels?

On the tire side of things I was wondering two things:

1. Replacing my steel wheels with lighter weight alloy/Mag/whatever material wheels will give better MPG?

2. Replacing my stock tires with thinner ones give better MPG?

Is the tire/wheel a part of a car that can greatly increase MPG when an optimum tire/wheel combo is used? Or will the gains be so small as to make this irrelevent?
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Old 04-06-2007, 01:46 PM   #2
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The tires will likely yield better gains. But.. you need to either test the tires relative to each other, or find the results of someone who has. For instance, on 14" tires, there can be a huge difference in the rolling resistance coefficient, Crr. Meaning that if you had the worst tires on, the car would require twice as much energy to roll compared to the best tires. Around town the impact is much more than at highway speeds, where aerodynamic drag dominates instead of rolling drag. Neway, as long as you buy the proper tires, they'll give a much bigger bang for you buck than lighter wheels.

For instance, a 2,000lb car weighs ~10,000N (Newtons). Rolling resistance is just the Crr times the weight. Lets say lighter wheels drop 100lbs off compared to stock, and better tires drop the Crr from .010 to .007 since you probably don't have the worst tires on, but not the best either.

Stock rolling force is 2,000lbs*.010=200lbs
Light wheels- 1,900lbs*.010=190lbs
Better tires- 2,000lbs*.007=140lbs

Better tires will also probably be cheaper, especially if you need to replace the stock ones. This won't directly translate into 30% better efficiency via rolling resistance, since you're also reducing engine efficiency. But it will be a significant improvement nonetheless.
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Old 04-06-2007, 01:51 PM   #3
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The tires will likely yield better gains. But.. you need to either test the tires relative to each other, or find the results of someone who has. For instance, on 14" tires, there can be a huge difference in the rolling resistance coefficient, Crr. Meaning that if you had the worst tires on, the car would require twice as much energy to roll compared to the best tires. Around town the impact is much more than at highway speeds, where aerodynamic drag dominates instead of rolling drag. Neway, as long as you buy the proper tires, they'll give a much bigger bang for you buck than lighter wheels.

For instance, a 2,000lb car weighs ~10,000N (Newtons). Rolling resistance is just the Crr times the weight. Lets say lighter wheels drop 100lbs off compared to stock, and better tires drop the Crr from .010 to .007 since you probably don't have the worst tires on, but not the best either.

Stock rolling force is 2,000N*.010=200N
Light wheels- 1,900N*.010=190N
Better tires- 2,000N*.007=140N

Better tires will also probably be cheaper, especially if you need to replace the stock ones. This won't directly translate into 30% better efficiency via rolling resistance, since you're also reducing engine efficiency. But it will be a significant improvement nonetheless.
Thanks for the response.....lets say for the sake of argument that I have the best tires to be found, giving the least RR money can buy, and I over inflate the tires to squeeze even more MPG....so having done that, would it be beneficial to find lighter wheels? Or will the increase in MPG be insignificant?

I do agree with you, that a good tire can do lots for increasing MPG over a bad one....
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Old 04-06-2007, 02:04 PM   #4
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When the weather turns nicer I'm going to try an A-B-A test with the two 'donut' tires I have for my car. The little donuts can be inflated to 60psi and are real skinny.

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Old 04-06-2007, 02:50 PM   #5
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Originally Posted by MorningGaser View Post
Thanks for the response.....lets say for the sake of argument that I have the best tires to be found, giving the least RR money can buy, and I over inflate the tires to squeeze even more MPG....so having done that, would it be beneficial to find lighter wheels? Or will the increase in MPG be insignificant?

I do agree with you, that a good tire can do lots for increasing MPG over a bad one....
Np. Provided your tires were the best possible, it may, or may not be beneficial to shave off ~100lbs depending on your outlook. Light wheels will probably be very expensive, and if most of your driving is at ~55mph, rolling resistance may be ~40% of the energy needed, and aerodynamic resistance may be ~60%. So, if you drop 100lbs, it would decrease the energy needed by ~2%, and increase fuel efficiency by ~1%. Now, depending on lifetime cost, this mod may, or may not pay itself back. If you average 45mpg over 250k miles, with gas at $3/gal, you'll only save ~$150 by increasing efficiency 1% w/ lighter wheels. So unless you rack up nearly a million miles on those wheels, or 500k w/ gas at $6/gal, it wouldn't be worthwhile financially.

That money would definitely better spent on synthetic engine and gear oil, aerodynamic modifications, or changes in gearing imo. You could also drop hundreds of pounds for free by removing unneeded items/weight from the car.
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Old 04-06-2007, 04:48 PM   #6
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"That money would definitely better spent on synthetic engine and gear oil, aerodynamic modifications, or changes in gearing imo. You could also drop hundreds of pounds for free by removing unneeded items/weight from the car."
I have done the synthetic engine oil, but not the gear oil in my 5 speed manual trans. Does the gear oil really make a difference?
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Old 04-06-2007, 05:20 PM   #7
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I think so, because efficiency gains are cumulative. Assuming a 3% synthetic increase for both oils with 20% engine and 80% transmission efficiency.

stock-.2*.8=.16
engine oil-.206*.8=.1648
engine and gear oil-.206*.824=.169744

Just oil results in a 3% gain, and gear oil and oil results in 6.09% gain. Now, synthetic gear oil compared to normal may result in more or less than a 3% difference, just like engine oil. But the point is that changes are cumulative and the total is greater than the sum of it's parts, iirc one of the lube companies did a test on a fully loaded big rig and saw a 10% increase in efficiency with all synthetic lubes, including bearing/axle grease. Less wear is just a bonus, the increase in efficiency and longer intervals between changes should make up for the price increase on an average car iirc.
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Old 04-06-2007, 07:23 PM   #8
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Quote:
Originally Posted by MorningGaser View Post

1. Replacing my steel wheels with lighter weight alloy/Mag/whatever material wheels will give better MPG?
Lighter wheels might help more in city driving. Drag racers will tell ya rotational weight is very important, losing 10 pounds off the wheels/ driveshaft/ect. is equal to losing 100 pounds of static weight. So in a lot of stop and go lighter wheels might pay off.
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Old 04-06-2007, 08:39 PM   #9
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Quote:
Originally Posted by Peakster View Post
When the weather turns nicer I'm going to try an A-B-A test with the two 'donut' tires I have for my car. The little donuts can be inflated to 60psi and are real skinny.
I look forward to this test
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Old 04-07-2007, 12:10 AM   #10
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FYI, on the smart fortwo, when the diesel version is equipped with 175/55-15 front tires and 195/50-15 rear tires (like I have), the fuel consumption goes up by 5.8% in the EU cycle fuel economy test (compared to the regular sizes of 145/65-15 front and 175/55-15 rear). That sounds like a lot, but it's only 0.2 L//100 km (and the wider tires and wheels have huge benefits in terms of roadholding).

So losing 3 cm off the width of a front tire should make a measurable difference. Also, low rolling resistance tires like the Continental Eco Contact or Michelin Energy should help as well.

Would you recoup the cost of new tires and wheels if your present ones don't need replacing? No way!
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