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Author Topic: FtIbs?  (Read 10288 times)

Offline blazer74

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Re: FtIbs?
« Reply #15 on: November 13, 2016, 12:01:52 pm »
4.1 revolutions of the axle shaft vs 1 revolution of the drive shaft input.  Higher engine rpm for same rotation speed of the tire gives you more power and torque at the wheels with the same size tire as with the 3.08 which is 3.08 turns of axle shaft to 1 turn of the driveshaft.   Hence 3.08:1 (3.08 to 1)ratio or 4.10:1 ratio

35 inch tire with 4.10 gears would be say 2200 rpm @ 55 mph vs say 35 inch tire with 3.08 gears would be 1800 rpm pm for the same vehicle speed. Higher rpm for a given speed gives you more power from the engine.

When you go with a bigger tire and the same gear ratio you are effectively lowering the gear ratio causing lower rpm and power from the engine for any given speed.
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Offline Irish_Alley

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Re: FtIbs?
« Reply #16 on: November 13, 2016, 02:58:55 pm »

yellow is better mpgs (usually) black is generally close to stock and blue is more power
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Offline 1967KaiserM715

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Re: FtIbs?
« Reply #17 on: November 13, 2016, 04:04:40 pm »
What do those numbers represent? Not the ratio or tire diameter.


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Offline Irish_Alley

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Re: FtIbs?
« Reply #18 on: November 13, 2016, 06:11:58 pm »
your rpm. find what tires you want then go over to you hit the yellow, black or blue then go up. it will then tell you what ratio you would need with a 1:1 transmission (no overdrive). if you have a OD trans then just take that rpm x it by .073
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Offline blazer74

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Re: FtIbs?
« Reply #19 on: November 13, 2016, 07:30:50 pm »
 The chart is based on 65 mph
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Offline 1967KaiserM715

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Re: FtIbs?
« Reply #20 on: November 13, 2016, 08:51:40 pm »
Ok thought it was rpm, but I guess I needed to know what speed as well, pretty sure I had calculated a 2100ish rpm at 55 with 31's and 3.42, so it caught me by surprise a little to see I was a few hundred rpm off.


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Offline blazer74

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Re: FtIbs?
« Reply #21 on: November 13, 2016, 09:14:12 pm »
Yes rpm is the result of size and gear ratio,  65 mph is the constant.
Also with automatics the Rpm's will be slightly higher  due to internal slippage.
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Offline Zac.richardson1

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Re: FtIbs?
« Reply #22 on: November 15, 2016, 09:17:13 am »
 what does that 1.02 difference mean
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Offline blazer74

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Re: FtIbs?
« Reply #23 on: November 15, 2016, 10:34:34 am »
Quote from: Zac.richardson1 on November 15, 2016, 09:17:13 am
what does that 1.02 difference mean

Diff between 4.10 and 3.08 is 1.02 which means with 4.10 gears the tires will turn 1.02 times more per 1 revolution of the driveshaft than with 3.08 gears for the same vehicle speed.

Which means higher engine rpm for the same speed resulting in more power at the wheels and less MPH.

Changing to a larger diameter  tire size has the same effect as numerically lowering your gear ratio( less rpm for a given speed and power) and vice versa.
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Offline Irish_Alley

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Re: FtIbs?
« Reply #24 on: November 16, 2016, 07:34:40 am »
Quote from: blazer74 on November 15, 2016, 10:34:34 am

Changing to a larger diameter  tire size has the same effect as numerically lowering your gear ratio( less rpm for a given speed and power) and vice versa.

most time when people put 38+ they change their ratio. if they just drive on the streets 4.10 is common if they want to go mudding (a lot and generally dont drive on the streets) 4.56 is their choice
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Offline roundhouse

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FtIbs?
« Reply #25 on: November 18, 2016, 05:25:04 am »
38s on a stock 10 or 12 bolt axle will kill the axle in a few months
The 10 and 12 bolt axles are not strong enough to hold up to stock tires


I bet if you look closely at the junkyard that has all the squares
Quite a few will be mostly complete except for the back axle is missing
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« Last Edit: November 18, 2016, 05:26:38 am by roundhouse »
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Offline Zac.richardson1

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Re: FtIbs?
« Reply #26 on: November 23, 2016, 01:03:06 am »
Ok I guess that's an answer. Your not wrong either but I was kinda asking in less of a mathematical sense lol
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