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Old 12-18-2007, 05:09 PM
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rear ratio & tire size

Hi
If a car comes with 4.88 gears and has (lets say) 26" tires and you put (lets say) 30" tires Then what ratio is it comparable to. Is there a formula for this?
Just wondering, and how do u know what speedo gear to run?
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Old 12-18-2007, 05:55 PM
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rear ratio & tire size

i'd wonder what formula you use for breaking ,over the ratio and tire size.
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Old 12-18-2007, 06:20 PM
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4.23 rear end ratio http://4lo.com/calc/geartire.htm
Shane
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Old 12-18-2007, 06:45 PM
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Hey Chevrolet 4X4---A BIG THANK YOU!!!!
That chart is just what I was looking for.
Merry Christmas
Gary
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Old 12-18-2007, 07:08 PM
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Your welcome Gary, there are several other useful calculators on that site,What are you running 4.88s in?
Merry Christmas to you too
Shane
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Old 12-18-2007, 07:45 PM
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My 26T it had a 4cylwith OD,but I'm putting a sbc in over the winter and want to have some fun with the 4.88'S before I change ratios but I'm going to need a pretty tall tire if I want to go anywhere further than 10 or 20 miles I have 255/70/15 now and they are about 29" I would like to find something 31or32" but so far all I see are truck tires wit aggresive tread or something that I can't afford ($200 and up each)
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Old 12-18-2007, 08:17 PM
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As far as I know the gear ratio are all based on 29'' tall tires,Not 100% sure but I always heard that. Let's say 411 is 411 with 29'' tires,Not with 26''
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Old 12-18-2007, 09:08 PM
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I wouldn't be too sure about that,I have 255/70/15 and they are 29"----How many new cars come with something that big? What about the early camaros and mustangs and a lot of early muscle cars came with 14" wheels. Their tires were no were near that tall
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Old 12-18-2007, 09:27 PM
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Quote:
Originally Posted by yragat
I wouldn't be too sure about that,I have 255/70/15 and they are 29"----How many new cars come with something that big? What about the early camaros and mustangs and a lot of early muscle cars came with 14" wheels. Their tires were no were near that tall
That's why I put not 100% sure!!!
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Old 12-18-2007, 10:17 PM
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Gear ratio is the comparison and relationship between the ring gear and the pinion gear. Example: 44 tooth ring gear and 11 tooth pinion is 4.0:1 . It has nothing to do with tire size.
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Old 12-18-2007, 11:18 PM
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O yes it does. A taller tire will equate into a lower numerical ratio. For instance if u have 4.88 then the tire turns over i time for every time the driveshaft turns 4.88 times,but if you go taller with the tire then it covers more footage than a shorter one
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Old 12-19-2007, 07:17 PM
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You multiply the gear ratio by (the old tire diameter divided by the new tire diameter). This will tell you what gear ratio it would "feel like" if you had changed gears instead of tires.

4.88 x 26/30 = 4.23

You know it will feel like it has less gear, so you set up the tire diameter fraction so it's less than one.
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Old 12-19-2007, 08:42 PM
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what do u mean by tire diameter fraction?
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Old 12-20-2007, 01:46 PM
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Old tire diameter divided by new tire diameter. Like Jimfulco posted.

If you have 4.88s on 26" tires, then go to 30" tires, you can find the new "effective" ratio like he said: 4.88 x 26/30 = 4.23.

You're not actually changing the ratio of course, but your highway RPMs would change the same as if you swapped gears to 4.23s
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Old 12-20-2007, 04:58 PM
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Just me being silly , but it doesn't matter what sized tire you put on the axle .... 3.43:1 is still 3.43:1 and 4.11:1 is still 4.11:1 .... those gears and the ratio will not change due to a bigger/smaller tire. You WILL cover more ground in the same :1 turn , so you will go faster than before at the same RPMs , but your 4.11s are still 4.11s.

I think....
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