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Old 01-26-2009, 06:15 PM   #1 (permalink)
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Wheels - 13 or 14 for better efficiency?

The 14 inch will reduce rpm by 4% but the 13 weighs 5 or 6 pounds less with the tire on, for more than 20 pounds less for all four. And lighter wheels should give a smoother ride.

The tough part is the car will be used for both long road trips and in town driving. Maybe 2 sets of wheels are called for?

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Old 01-26-2009, 06:24 PM   #2 (permalink)
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Quote:
Originally Posted by hypermiler01 View Post
The 14 inch will reduce rpm by 4% but the 13 weighs 5 or 6 pounds less with the tire on, for more than 20 pounds less for all four. And lighter wheels should give a smoother ride.

The tough part is the car will be used for both long road trips and in town driving. Maybe 2 sets of wheels are called for?
I'm wondering what the weight difference is in a taller profile tire for the 13" rims...
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Old 01-26-2009, 07:10 PM   #3 (permalink)
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The 13 inch tires weigh about 1 or 2 pounds less, the 13 wheels weigh about 4 pounds less, so almost 6 pounds less total, times 4, plus a shorter lever arm for less inertial resistance. Plus I can get 13 tires in 155 width, opposed to 175 for the 14, so it may even have less rolling and air resistance.

Pick up 6 pounds in your hand and wave it around in the air. Removing that weight also gives better suspension control.

Maybe I just answered my own question, but I have no measured data. Anyone who does?
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Old 01-26-2009, 08:27 PM   #4 (permalink)
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Depending on your tire sizes, you can get 155/80/13's which are the highest standard profile for a 13" rim, which equal 22.8 diameter.

This is roughly equivalent to 195/55, 185/60, 175/65, 165/65, and 165/70R14 Tires, none of which are more than .5 inches (nominal measurement, each tire mfr and brand will vary) larger or smaller than the 155/80R13.

The 155/80 is close to equal with a 175/70R13, a common tire size on small cars.
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Old 01-27-2009, 01:00 PM   #5 (permalink)
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13" wheels will give you a selection of tires that are narrower than those made for 14" wheels. That will help decrease rolling resistance.
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Old 01-27-2009, 02:46 PM   #6 (permalink)
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I switched from 185/60-14 to 155/80-13. The weight savings was 6 lbs per corner, but more significantly, the tire weight went down by 4 lbs each. When I calculated the inertia difference, 80% of the inertia is in the tire as the weight is concentrated further from the rotational axis. I calculated that this change would requrie about 20 lb-ft less torque for the same acceleration.

The results, the car accelerates way easier and you can really keep your foot out of it. It's very noticeable around town, less so on the highway. As far as FE, I don't have any numbers yet as the snow started falling and I put the heavier steel wheels and snow tires on.
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Old 01-27-2009, 03:02 PM   #7 (permalink)
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Don't forget to consider aero as well (not speaking only of the tire width, as Christ mentioned).

Do you have identical smooth wheel covers for the 13 & 14's?

http://ecomodder.com/forum/showthrea...-6-a-4368.html
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Old 01-27-2009, 03:10 PM   #8 (permalink)
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175/70R13 is what I have now, and there is a pretty good selection of tires for that size. The only 155/80R13 that I find is Khumo Solus KR21.

I guess changing the final drive is a better way to lower rpm on the highway than putting taller, heavier tires on.

I will have full skirts, front and rear, so not sure how much extra improvement moons would make if they are not in the airstream at all.
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Old 01-27-2009, 04:39 PM   #9 (permalink)
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Quote:
Originally Posted by wagonman76 View Post
13" wheels will give you a selection of tires that are narrower than those made for 14" wheels. That will help decrease rolling resistance.
Don't be too sure about that.

Where's the evidence that skinnier is always lower r.r.?
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Old 01-27-2009, 04:57 PM   #10 (permalink)
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I remember a basjoos post where he discussed this. He had wide tires that were lower rolling resistance than skinny tires. In a stock car, using the skinny tires was more efficient because they create less aero-drag; however, in his well-skirted car, the tires did not affect airflow very much, so wider, lower rolling resistance tires were more fuel efficient. You'd really need to look at the specs of two given tires in comparison rather than just assuming narrower is more or less rolling resistant.

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