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Old 09-19-2008, 09:12 PM   #1 (permalink)
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stinky - '97 geo metro lsi
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tire diameter

This question is kind of for metrompg but would like anybodys input. Has anybody checked out the difference in fuel economy in say the 12, 13 and 14, maybe even bigger ties all at the SAME RPM, for example, 12 inch tire at 3000 rpm and 55 mph versus 14 in tire at 3000 rpm and 55 mph, especially since the 12 inch should be alot lighter and narrower, even less sidewall to flex. ( assuming we arnt going into expensive low profiles.)

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Old 09-19-2008, 09:52 PM   #2 (permalink)
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The inside diameter (12, 13, 14, 15, 16 ,17) doesn't really matter. It's the outside diameter that counts!

For instance, a 185/65-14 tire has the same outside diameter as a 205/40-17 tire, hence the same revolutions per mile, and the same RPM. This is a common upgrade for Honda CiViC owners - going to 205/40-17.

EDIT

Sorry! I just reread your post.

I guess we were using the same hypothetical...

So, using my example, the question becomes: Will a narrower, normal profile 185/65-14 tire give better FE than an identical outside diameter, but wider, low profile 205/40-17 tire?

I would *guess* the answer is yes... but that's just an 'educated' opinion based on participating on several Honda blogs over the years! Lower FE is a common complaint with Honda CiViC owners that switch to 17s.

Nobody gets the mileage I do with a B16A2 - I gotta be World Champ - and I credit a large part of my success to running a lighter, narrower, tire than 'normal' @ 50 PSI, and an even lighter right foot!
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Old 09-19-2008, 10:19 PM   #3 (permalink)
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Does anybody use common sense around here. Im sorry to be rude but im in a bad mood. the tires are just standard tires ( i really hope i dont have to waste even more time getting every tire size for each tire) yes the diameters get bigger as you go up in size, but as the tire gets bigger, u change gearing to keep the rpms equal. therefore if one tire seems to be better than another, you would change gearing till u ended up with the best tire, and the best gearing.
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Old 09-19-2008, 10:24 PM   #4 (permalink)
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Quote:
Originally Posted by guitarterry View Post
Im sorry to be rude but im in a bad mood...

(Y)es the diameters get bigger as you go up in size, but...
Sorry you're in "a bad mood", but...

Yes, you are going to have to get every tire size for each tire and run the numbers through a 'tire calculator', such as: Tire size calculator

As the tire size goes up, you can actually end up with a smaller diameter tire, e.g. higher RPM at the same speed - not less!
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Old 09-19-2008, 10:30 PM   #5 (permalink)
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after reading mertompg's tranny swap page i noticed that 145/82/12 tires in fifth and 155/80/13 in forth are almost the same. find a way to make it even closer and i could test.
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Old 09-19-2008, 10:35 PM   #6 (permalink)
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Did he adjust his figures for the speedo being off almost 8% (slow) with the new tire size?
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Old 09-19-2008, 10:48 PM   #7 (permalink)
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Black, metro is an expert of course he did, i was in a bad mood cause im just trying to use common sense and generalitys. i dont care if some special 17 inch tire is smaller than a 12 inch tire. NORMALLY, NORMALLY, NORMALLY, 12 inches are narrower then 13, 13 is narrower than a 14, etc etc. NORMALLY a 12 is smaller diameter than a 13, a 13 is smaller diameter than 14.
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Old 09-19-2008, 11:02 PM   #8 (permalink)
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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.
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Old 09-19-2008, 11:13 PM   #9 (permalink)
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If you are wondering how wide or skinny of a tire you can run on a particular rim size you can figure that out from those calculations too. Most tires can be stretched a little bit to fit on a slightly wider wheel than they were intended for.

To determine if a tire will fit on a specific rim you have to know the width of the rim. Then just take the width of the tire say a 155/80/13 again. Divide 155 / 25.4 = 6.1 inches of tread width. These should fit on 5.5 - 6.5 inch wide rims.


Also note that while the widths of rims generally go up with increased diameter, the diameter of every 12 inch wheel will be 12 inches, every 13 is 13 etc. The overall diameter of the wheel & tire combo is only changed by the tire.
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Old 09-19-2008, 11:29 PM   #10 (permalink)
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Can anybody see me pulling my hair. DO NOT CARE. Let me try an exteme to pound this into u guys head. I take a 5 inch diameter tire. YES 5 inch. put it on the car. Change the gearing in the car to where it turns 3000 rpm at 55 mph. find out the mpgs. then i put a 100 inch tire on, yes 100 inch, change the gearing to where rpm is 3000 at 55 mph. find out the mpg,s compare the results.

now for the people who are going so say a 5 inch tire wont work cause of ground clearence, do not care. I know that lower rpms are better, thats not the question so, do not care. tires can get smaller as rim gets bigger, do not care. NORMAL tires get wider as the diameter get bigger, so special (code name, expensive) tires that get narrower as the diameter goes up, do not care. What i am trying to compare is something (but not limited to) 155/80/13 tires to 145/82/12 geared to make the rpms and speed equal. I hope that i am getting specific enough.

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