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Old 09-13-2008, 11:04 AM   #1 (permalink)
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Question magwheels and tires

i just want a confirmation if my understanding is correct about mags and tires.

it doesn't matter if you are using 13s or 18s because:

smaller diameter overall=one revolution of your tire covers less ground but effectively spins faster per minute
bigger diameter overall=one revolution of your tire covers more ground but effectively spins slower per minute

what will affect economy will be the width of the tires. the wider you go, the higher the friction your engine's power will have to overcome. more contact patch=more friction=more energy needed to overcome that friction.

is this true?

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Old 09-13-2008, 11:29 AM   #2 (permalink)
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the main reason people tend to go for smaller rims is that they tend to be narrower then larger around rims, and they tend to be lighter weight as well, you are correct that a larger around tire will need to be spun slower for a given speed, allowing your engine to turn slower as well.
the only reason to go with wider tires is if vehicle weight goes up, as wider tires normally have a higher load range, and just like on a bicycle, you want the narrowest tires you can get away with, and a narrow tire is going to need to be at a higher pressure to give the same amount of support.
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Old 09-13-2008, 02:08 PM   #3 (permalink)
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Also remember that as a rim increases in diameter, it gets heavier. If an 18" rim and tire is same overall height as a 14" rim and tire, the 18" will weigh more.
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Old 09-13-2008, 02:36 PM   #4 (permalink)
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Remember also that a larger diameter wheel will move the concentration of mass closer to the outside of the wheel. I have seen lightweight wheels in up to 20 inch sizes (cost is very high) but the tire weight is most of the assembly then. Increasing the power required to get the wheel and tire assembly moving/stopped.
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Old 09-13-2008, 03:31 PM   #5 (permalink)
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Contact patch is dependant on air pressure. Wider tires are less aero. Heavy tires lower MPG. The ultimate tire is the skinniest, lightest tire aired up to max. The trade off is ride and safety.
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Old 09-13-2008, 03:38 PM   #6 (permalink)
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About overall final tire diameter -
Remember that if you change the overall diameter you will need to change your speedometer/odometer for them to be accurate. Or ignore them and use a GPS.

That really doesn't seem practical to me for daily use over the long run.

But there's usually a variety of tire sizes you could use for any given car. They will have different widths and/or wheel rim sizes and profiles (aspect ratio). But there will be several different sizes with the same diameter/circumference that you can use on the car.

Out of those available options, I think the narrowest tire would give the best fuel economy, all other things being equal.

Last summer I went from 197/75-14's to 205/65-15's (all wheels were/are steelies). Same outer diameter. If you look at my fuel log, this summer I haven't been able to match the FE numbers I got last summer. My commute route is different but the overall distance is similar. I suspect my bigger, heavier tires and wheels.
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Old 09-14-2008, 08:58 AM   #7 (permalink)
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Upgrading rims for mpg purposes really won't help much. If you stick with stock size and go with lighter rims and skinnier tires you may see an increase.

I've personally upgraded my rims from 16" 18lb stock rims to 17" 14lb performance rims, but I had to go with a slightly wider tire. It has had a negative impact on my mpg.
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Old 09-14-2008, 09:50 AM   #8 (permalink)
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Quote:
Originally Posted by c0da View Post
Upgrading rims for mpg purposes really won't help much. If you stick with stock size and go with lighter rims and skinnier tires you may see an increase.

I've personally upgraded my rims from 16" 18lb stock rims to 17" 14lb performance rims, but I had to go with a slightly wider tire. It has had a negative impact on my mpg.
Are you correcting for the difference in size between your new tires and the stock ones? If they're taller (and there's a good chance they are), you are traveling slightly more miles than your odometer says you are.

For example, I actually run a slightly wider (and taller) than stock tire, with steel wheels. This means I have to multiply my miles by 1.02 (since the circumference is 2% greater than stock), which means my actual mpg is a bit higher than strictly dividing miles by gallons.

In addition, a lot depends on the mix of driving you do. Stop-and-go driving benefits most from lightened wheels and tires, whereas interstate driving isn't hindered by- and might actually benefit from- steel wheels.
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Old 09-14-2008, 09:54 AM   #9 (permalink)
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In the classical literature, tall, skinny wheels have the lowest rolling resistance. Wide tires are bad for air drag, but the radial-belted construction may have changed the traditional advice. Unfortunately, the data is proprietary information, so about all we can do is look up the LRR tires, such as the Bridgestone Potenza RE92 and choose light wheels to suit them. Larger overall diameter will change the speed and mileage readings, but also reduce engine RPM, which usually helps.
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Old 09-14-2008, 10:27 AM   #10 (permalink)
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whokilledthejams,

Even with the compensation I still have about a 3mpg drop every tank. The tires I have are not only wider, but they are stickier performance tires. I'm looking for LRR for the next ones I get.

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