Thread: tire diameter
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Old 09-20-2008, 10:40 AM   #20 (permalink)
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stinky - '97 geo metro lsi
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Originally Posted by 1.5Ldave View Post
The rim size wont make a significant difference at all. Its the overall diameter of the tire that needs to be considered. A larger overall diameter will give you lower rpm by changing the final ratio. This will also effect acceleration.

For those who dont know how to decipher the tire coding a 155/80/13 tire would be 155 millimeters wide, 80 is the aspect ratio of the sidewall, and 13 is the size of the rim.

To calculate total diameter you multiply the width times the aspect ratio
155 x 0.80 = 124 mm.
Divide by 25.4 (#of mm per inch)
124 / 25.4 = 4.88 inches of sidewall.
Multiply by 2 (sidewall above and below rim)
4.88 x 2 = 9.76 inches
Add the size of the rim (13) for a diameter of 22.76 inches for the 155/80/13

Now well run the 145/82/12
145 x 0.82 = 118.9 mm
118.9 / 25.4 = 4.68 in
4.68 x 2 = 9.36 in
12 + 9.36 = 21.36 inch diameter

This explains the better mileage since the 155/80/13, even though almost a 1/2 inch wider, is also almost 1.5 inches taller. Lower rpm at cruising speed = less gas used. A taller tire is the easiest way to change the final drive of your car, though not the most effective.
ok dave you are close.
we agree lower rpm at cruising speed = less gas used
My original question states the rpms are equal. (i know it does I wrote it)
so we take the 12 in 21.36 diameter tire and gear the tranny to be going 55 mph at 2000 rpms in the engine, then the 13 in 22.76 diameter tire going 55 mph at 2000 rpms in the engine. wow that is what i wrote in my original question. we then say what effect will the lowering of height and mass have using the 12,s and why arent we using the 12's if the apparent FE advantages are so apparent.
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