Regearing made easy. - Off-Road Forums & Discussion Groups
Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

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post #1 of (permalink) Old 02-17-2007, 08:20 AM Thread Starter
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Regearing made easy.

[img]/ubbthreads/images/graemlins/smile.gif[/img] I noticed a post on the Main Index from a newbie who needed to know the what and the how that goes along with choosing a new gear ratio to go with the new lift and the new tires. It has been a while since I posted this, so just in case there is someone left on the planet who doesn't know how to do the calculations for regearing, here is a refresher: It is simple (Velocity Squared) divided by 2G. Oops.... that isn't it, that's how you clculate the momentum head for a large axial flow pump.. [img]/ubbthreads/images/graemlins/crazy.gif[/img]... OH WAIT! ... HERE IT IS...!!!! [img]/ubbthreads/images/graemlins/crazy.gif[/img] You find the TURNS PER MILE of each tire, the old ones and the new ones. You can find TURNS PER MILE by looking in the tire dealer's catalog, OR ....you can find it by measuring the circumference of the tire in inches, divinding by 12 to get feet and divide that into 5280, the number of feet per mile. So that would give you the number of turns the tire makes per mile. So you do that for the old tires and for the new tires. THEN you check your existing gearing and whatever ratio you have now, multiply that by the TURNS PER MILE of the existing tires. So it would be something like 715 TURNS PER MILE X 3.54 (your ratio) which would give you a number like 2531. Now that number 2531 JUST HAPPENS TO BE the driveshaft RPM at sixty miles per hour..... WHAT A COINCIDENCE! So....... now stay with me here ..... if you take 2531 and DIVIDE it by the TURNS PER MILE of the bigger tires, the answer you get will be the NEW GEAR RATIO. So .....TURNS PER MILE (little tires) X Gear Ratio (existing) = TURNS PER MILE (big tires) X Gear Ratio (new)... [img]/ubbthreads/images/graemlins/smile.gif[/img] [img]/ubbthreads/images/graemlins/wink.gif[/img] TPM X Ratio(old) = TPM X Ratio(new) [img]/ubbthreads/images/graemlins/cool.gif[/img] And here you thought you'd never use any of that sophisticated high skool math. [img]/ubbthreads/images/graemlins/crazy.gif[/img] Once more... TPM X Ratio = TPM X Ratio
..... The idea is to get the driveline RPM at sixty miles per hour with the new gears and tires to be the same as it was with the original gears and tires at sixty miles per hour. [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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post #2 of (permalink) Old 02-17-2007, 09:20 AM
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Re: Regearing made easy.

Simple? Wosers Dave, you've got a demented way of simplifying things so early on a Saturday morning.... [img]/ubbthreads/images/graemlins/thud.gif[/img]

How about putting that in a simple formula for us dyslexics who get lost in verbage? [img]/ubbthreads/images/graemlins/wall.gif[/img]
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post #3 of (permalink) Old 02-17-2007, 09:55 AM
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Re: Regearing made easy.

Check Gear Ratio / Engine RPM Calculator for a good calculator.
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post #4 of (permalink) Old 02-17-2007, 10:11 AM
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Re: Regearing made easy.

Yeah, Dave - you make it WAY too complicated. Inches in a foot and feet per mile will cancel out if you simplify the equation before plugging in numbers.

Tire dimensions can be in inches, meters, feet, miles or furlongs or fathoms. It makes no difference as long as the same units are used for both tires they cancel out too. How about:

(New tire circumference / Old tire circumference) X Current axel ratio = New axel ratio

Or:

(New tire diameter / Old tire diameter) X Current axel ratio = New axel ratio

Those numbers come right off the sidewall. No, it's not precise, but it's all approximations anyway, because of the limited number of ratios available.
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post #5 of (permalink) Old 02-17-2007, 10:17 AM
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Re: Regearing made easy.

[ QUOTE ]
t makes no difference as long as the same units are used for both tires they cancel out too.

[/ QUOTE ]

Ohhhh..Ohhhh... Do it in Gunter's Chain! It's more Jeeplike!
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post #6 of (permalink) Old 02-17-2007, 02:16 PM
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Re: Regearing made easy.

...i prefer all my units to be in Angstroms

[img]/ubbthreads/images/graemlins/thud.gif[/img] [img]/ubbthreads/images/graemlins/thud.gif[/img]
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post #7 of (permalink) Old 02-17-2007, 09:30 PM Thread Starter
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Re: Regearing made easy.

[img]/ubbthreads/images/graemlins/crazy.gif[/img] CRAPO! I thought I was doing a: GOOD THING ????

I tried to keep it simple. [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/crazy.gif[/img] How's this!!! TURNS PER MILE(old tires) X GEAR RATIO(old gears)divided by TURNS PER MILE(new tires) = new gear ratio

719 X 3.54 divided by 619 = 4.11 Gee...he needs a four-eleven gear. [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/tongue.gif[/img] [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/tongue.gif[/img] [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/tongue.gif[/img] [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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post #8 of (permalink) Old 02-17-2007, 09:42 PM
 
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Re: Regearing made easy.

That's good.

But what if you're of the opinion, as I am, that a jeep comes from the factory with not enough gear period. I think they geared them for fuel economy and low rpms, and when I go from a 28" tire to a 33" tire, which is only an 18% increase in diameter, I want more than to just match the 3.3 rear end ratio the factor put in. The formula would tell me to use 3.90 gears, however I'd rather have 4.56's which are a 38% increase in ratio so that I have some ability to climb an incline or drive into the wind in overdrive.

So yes, the formula is great for matching the original gear ratio, but I say question whether you liked the original ratio, and what rpm would you like to hit at 60 mph.
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post #9 of (permalink) Old 02-17-2007, 11:04 PM
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Re: Regearing made easy.

Or use percentages -

33's are 3" larger than 30's. That's 10%!

That's a 10% DECREASE in RPM at whatever speed you are concerned about.

No need to know gear ratios, overdrive ratios etc to do that.

If you know your axle ratio, then use that percentage to figure the gears you want.

KISS!
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post #10 of (permalink) Old 02-18-2007, 06:14 AM Thread Starter
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Re: Regearing made easy.

[img]/ubbthreads/images/graemlins/tongue.gif[/img] You make a superb point about possibly wanting to deviate one way or the other, rather than just robotically duplicating what the original factory RPM was at sixty miles per hour. IN FACT, just bumping up to a bigger tire size can introduce losses that weren't there before. FOR EXAMPLE ..... you are pushing a bigger "footprint" with those massive mud hoggers, and that takes more power due to wind resistance and friction. PLUS, if you have lifted the Jeep, you are moving farther away from the "teacup effect" that low vehicles enjoy; an effect that reduces the air-under losses from wind resistance. So..... when your calcualtions show that you need a 3.9877 to one ratio to duplicate the factory RPM at sixty miles per hour, that's when you might want to apply some good old: "Kentucky Windage" to the equasion and say to yourself: "Hmmmmm....I'm going to go with the next available lower gear ratio and get me some more RPM!!! [img]/ubbthreads/images/graemlins/crazy.gif[/img] [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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