Visualizing a new tire size - Page 2 - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

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Old 05-11-2009, 09:56 AM   #11
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Whoops, let me try that again. Your stock tire is 25" according to Miata calculator, which would be about 178.5 pixels in diameter on that photo you wanted to use (assuming the length in the specs really is bumper to bumper). Your new tire is 26.3", which would be 187.8 pixels. The tires in that picture are 190 pixels or so (angled view of them made measuring precisely a little tough), so I'd say that the tires in that image are already just about the right size visually. Must be the 16" wheels. To double check we're talking about the same picture:

http://z.about.com/d/cars/1/0/5/0/1/...abbit_side.jpg
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Old 05-11-2009, 10:01 AM   #12
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Oh, I see. I don't expect those tires to be larger than 25"; the larger rim is usually used with a tire of the same outside diameter and that car looks stock.

I think my next step is taking good pictures and measurements of my actual car.
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Old 05-11-2009, 10:11 AM   #13
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Well that really shows the limit of my ability to measure things accurately via this method: too big by 5%.

Do you really think the diameter increasing by 1.3" is going to matter that much? That's only .65" less clearance to the wheel well after all.
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Old 05-11-2009, 10:34 AM   #14
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I'm more concerned that it will make the car look weird than I am about clearance, though clearance is a valid concern too. I'm not vain by any measure and don't need to look particularly good, but I do prefer to blend in and stay as invisible as possible.

I do think that 205-70-15 is probably the practical limit for either concern.

Some thoughts if I do change to that size:
  • An additional .65" ground clearance will have a tiny aerodynamic effect.
  • I'd probably get slightly longer treadwear
  • I'd probably have to inflate a little less and probably would have the same rolling resistance (if I had the same model tire in the new size) at the lower inflation
  • I'd reduce my highway RPM by ~150, not a huge gain...but probably worthwhile when I'm going to have to replace tires anyway.

Wouldn't you know it, for 2009 VW changed the final drive ratio in the car to a much taller ratio. The 2009 model, when I feed it into a gear calculator, runs 2650 RPM compared to my 3050 RPM at 70mph. They gained 1mpg highway, and somehow lost 1mpg city...I don't know how that worked. Maybe the EPA test kept the car in 1st a lot longer with the taller final drive.
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