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Old 09-05-2012, 07:23 PM   #31 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
If you can pull those from the junk yard new, all day for $10 or $20 each, its unbeatable.
Half were $5 the other half were $10 and I get a few cents for the rims.

Best part is they are the correct size for the car.

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Old 08-02-2014, 04:55 PM   #32 (permalink)
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From my experience yes, less weight and size means less rolling resistance. A simple formula is the max load of the tire should be equal to or greater than half of the GAWR front or rear on the Vehicle Certification Label, usually the rear is less on a fwd car. From there the smallest wheel with the smallest tire is best. You can easily find tire specs on tirerack.com This should be done before lowering the car as it may make it unnecessary to do. There may be a limit to how small of a wheel you can use to fit over brake calipers.
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Old 08-02-2014, 05:00 PM   #33 (permalink)
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cargo,
can you show us your tests that verify this.
it runs contrary to my tests and those of others.

smaller DIAMETER of tire results in HIGHER RPM at cruise speed. Higher rpms result in lower gas mileage.
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Old 08-02-2014, 06:21 PM   #34 (permalink)
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Quote:
Originally Posted by mcrews View Post
cargo,
can you show us your tests that verify this.
it runs contrary to my tests and those of others.

smaller DIAMETER of tire results in HIGHER RPM at cruise speed. Higher rpms result in lower gas mileage.
The test that I used was the simple fill up to first stop and then use a full tank of gas from a 0 trip odometer and then fill the same way and then divide the trip odometer by the gallons on the pump. This was on a Nissan Versa 4D 1.8SL CVT(which helped) going from a 225/45/17 (summer tires) to a 195/55/15 (snow tires) and consistently showed a difference of 3mpg exactly, 30 to 27 respectively. Even with a difference of 1.6" on a non CVT car you're only talking a difference of a couple hundred rpm which is negligible.
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Old 08-02-2014, 06:22 PM   #35 (permalink)
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Here's a calculator you may want to make a sticky

Tire Size, RPM, Speed, and Differential Ratio Calculator
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Old 08-02-2014, 07:38 PM   #36 (permalink)
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Quote:
Originally Posted by CargoBoatTails View Post
The test that I used was the simple fill up to first stop and then use a full tank of gas from a 0 trip odometer and then fill the same way and then divide the trip odometer by the gallons on the pump. This was on a Nissan Versa 4D 1.8SL CVT(which helped) going from a 225/45/17 (summer tires) to a 195/55/15 (snow tires) and consistently showed a difference of 3mpg exactly, 30 to 27 respectively. Even with a difference of 1.6" on a non CVT car you're only talking a difference of a couple hundred rpm which is negligible.
So was the difference in tire size factored in (6%)? The other 4% could easily be differences in rolling resistance differences of 2 very different tires
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Old 08-02-2014, 07:43 PM   #37 (permalink)
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Quote:
Originally Posted by CargoBoatTails View Post
The test that I used was the simple fill up to first stop and then use a full tank of gas from a 0 trip odometer and then fill the same way and then divide the trip odometer by the gallons on the pump. This was on a Nissan Versa 4D 1.8SL CVT(which helped) going from a 225/45/17 (summer tires) to a 195/55/15 (snow tires) and consistently showed a difference of 3mpg exactly, 30 to 27 respectively. Even with a difference of 1.6" on a non CVT car you're only talking a difference of a couple hundred rpm which is negligible.
It is REALLY important when doing tests (A-B-A ideally) that the variables are consistent.
1. The thread is about TIRE size. It is important to understand that it means ONLY tire size. 15 to 15 etc.
2. Tests comparing different size(14, 15, 16 etc) tires means you have different rims. so weight of each rim HAS to be taken into consideration.
3. Since the rims are different sizes, then there is the consideration for the movement of MASS in and out.
4. Finally, summer tire to winter tire has some weight difference, especially on different size tires and compound difference.
5. finally, you don't say if you are factoring for the odometer being off. YOu seem pretty clear that you don't. THe best way is with GPS mileage, it's always right regardless of tire size.

Cant really say that you 'tested' the results.

Again, MULTIPLE members have posted true tested results. A smaller TIRE, doesnot produce better mileage.
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Fourth: rear skirts and 30.4mpg on trip!
http://ecomodder.com/forum/showthrea...tml#post247938
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Old 08-02-2014, 07:54 PM   #38 (permalink)
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I wouldn't bother going with a smaller tire unless I was going to a smaller rim. Again the speedo error would be negligible and not in favor of the smaller tire anyway recording more miles, so yeah I was probably getting better than 3mpg improvement.

Here's another calculator for you:

http://tire-size-conversion.com/spee...r-calibration/
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Old 08-02-2014, 08:03 PM   #39 (permalink)
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Yes, the smaller tire would record more miles, about 6.1% more miles.
So if you travel 100 actual miles, and use 2 gallons of fuel, you would figure your mileage as 50mpg. With the smaller tires your odometer would show 106.1miles, even though you had only traveled 100 miles, and if you consumed the same 2 gallons, you would falsely calculate 53.05mpg
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Old 08-02-2014, 08:04 PM   #40 (permalink)
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A smaller circumference will reduce both speed and distance covered so to compensate you'd need to drive both faster and further, at least according to the speedo/odo.
Using GPS data is indeed the best way to resolve it.

Then your winter tires are softer, have higher side walls and narrower thread, these things will contribute to the FE. Even at the correct speed an distance those may compensate for the higher RPMs and higher rolling resistance from the shorter tire radius.

With so many things to consider it is near impossible to draw conclusions from this test alone.

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