Effective Gear Ratio
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Postmaster
Joined: Jan 2005
Posts: 2,753
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From: Phenix City, AL
Effective Gear Ratio
Just though I'd put this up. I was trying to figure my effective gear ratio after changing tires. Did some stumbling around and found a formula for it. I know it was driving me nuts figuring it out, so here it is for anybody else trying to figure it ou.
(Original Tire Dia / New Tire Dia) x Current Gear Ratio = Effective Gear Ratio
Just change the gear ratio number to figure out what gears you need to install to get the effective ratio you want.
(Original Tire Dia / New Tire Dia) x Current Gear Ratio = Effective Gear Ratio
Just change the gear ratio number to figure out what gears you need to install to get the effective ratio you want.
Posting Guru
Joined: Nov 2002
Posts: 2,440
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From: Ventura County, CA
BFR,
If that equation is accurate, your information should be posted in the first thread in this forum ("New Users... Read this First"...) along with all the other info on tire sizes with different lifts, etc..
Thanks for the info, it should be helpful to MANY who visit this forum...
Best,
DS
If that equation is accurate, your information should be posted in the first thread in this forum ("New Users... Read this First"...) along with all the other info on tire sizes with different lifts, etc..
Thanks for the info, it should be helpful to MANY who visit this forum...
Best,
DS
Elder User
Joined: Oct 2004
Posts: 587
Likes: 0
For most guys, we are wanting to keep the same relative ratio after a tire change so the computer will still drive the transmission the same as before, keeping the truck's operating RPM's in the right spots. If you change the formula up as shown below, it will give you the new ratio needed without all the data interpretation and inputs as origionally posted:
(New Tire Dia/ Original Tire Dia) x Current Gear Ratio = New Comparitive Gear Ratio
(New Tire Dia/ Original Tire Dia) x Current Gear Ratio = New Comparitive Gear Ratio
Senior User
Joined: Mar 2004
Posts: 324
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I see there are two different formulas here. And they both seem very simple. Let me try each formula and see what I get. Btw, I'm guessing that the original tires are about 32".
38"(New) / 32" (Original) = 1.1875
1.1875 x 4.88 (Current gears) = 5.79
5.79 = Effective Gear Ratio? Holy cow!
Or...
32" (Original) / 38" (New) = .8421
.8421 x 4.88 (Current gears) = 4.10
4.10 = Effective Gear Ratio?
I like the second result better. I'm I doing this correctly? Because as you can see the results differ greatly.
38"(New) / 32" (Original) = 1.1875
1.1875 x 4.88 (Current gears) = 5.79
5.79 = Effective Gear Ratio? Holy cow!
Or...
32" (Original) / 38" (New) = .8421
.8421 x 4.88 (Current gears) = 4.10
4.10 = Effective Gear Ratio?
I like the second result better. I'm I doing this correctly? Because as you can see the results differ greatly.
Cargo Master
Joined: Feb 2005
Posts: 2,837
Likes: 1
You guys are confusing the heck out of me. What I want to know is when I put a minor 4" lift with 35's onto my truck, what gearing should I go with to keep the ratio the same as with stock tires and 3.73's? I think the second one is what I'm looking for, but not sure. I'm sure you guys know what you're talking about, but how about an example so that those of us without the high IQs can figure it out.
Thanks,
Bob
Thanks,
Bob
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Postmaster
Joined: Jan 2005
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From: Phenix City, AL
Example follows:
Original tire 32"
New Tire 37"
Current Gears 4.56
(32 / 37) x 4.56 = Effective ratio
32 / 37 = .865
.865 x 4.56 = 3.94 effective ratio
Now, to figure out where you want to be just change the gear ratio number (in this case 4.56) to what you are thinking about putting in. The following example will be with 4.88 gears.
32 / 37 = .865
.865 x 4.88 = 4.22 effective gear ratio
The only thing that is variable is the gear ratio you have or want. Simply change that number to see what gears you need to install to get the effect you want. This EFFECT is relative to your original setup with original tires. So if your truck had 4.10 gears and stock tires and you want to maintain your original power, or have more, you change the gear number to see this effective gear ratio.
Original tire 32"
New Tire 37"
Current Gears 4.56
(32 / 37) x 4.56 = Effective ratio
32 / 37 = .865
.865 x 4.56 = 3.94 effective ratio
Now, to figure out where you want to be just change the gear ratio number (in this case 4.56) to what you are thinking about putting in. The following example will be with 4.88 gears.
32 / 37 = .865
.865 x 4.88 = 4.22 effective gear ratio
The only thing that is variable is the gear ratio you have or want. Simply change that number to see what gears you need to install to get the effect you want. This EFFECT is relative to your original setup with original tires. So if your truck had 4.10 gears and stock tires and you want to maintain your original power, or have more, you change the gear number to see this effective gear ratio.
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Postmaster
Joined: Jan 2005
Posts: 2,753
Likes: 3
From: Phenix City, AL
Robert here is what you are looking for.
Orignal tires on a SD are approx 31.5". You want 35's and have 3.73 gears. Sooo.
31.5 / 35 = .9
.9 x 3.73 = 3.36 effective ratio (currently)
Here's some changes to the setup. We keep the same .9 throughout, we're just changing gear ratios
.9 x 4.10 = 3.69 ratio
.9 x 4.30 = 3.87 ratio
.9 x 4.56 = 4.10 ratio
.9 x 4.88 = 4.39 ratio
You get the point. To keep your truck closest to the original level of performance with 35" tires, you would go with 4.10 gears. This would put you at an effective gear ratio of 3.69
Hope that helps a little.
Orignal tires on a SD are approx 31.5". You want 35's and have 3.73 gears. Sooo.
31.5 / 35 = .9
.9 x 3.73 = 3.36 effective ratio (currently)
Here's some changes to the setup. We keep the same .9 throughout, we're just changing gear ratios
.9 x 4.10 = 3.69 ratio
.9 x 4.30 = 3.87 ratio
.9 x 4.56 = 4.10 ratio
.9 x 4.88 = 4.39 ratio
You get the point. To keep your truck closest to the original level of performance with 35" tires, you would go with 4.10 gears. This would put you at an effective gear ratio of 3.69
Hope that helps a little.
Posting Guru
Joined: Mar 2005
Posts: 2,221
Likes: 5
Originally Posted by beemr
For most guys, we are wanting to keep the same relative ratio after a tire change so the computer will still drive the transmission the same as before, keeping the truck's operating RPM's in the right spots. If you change the formula up as shown below, it will give you the new ratio needed without all the data interpretation and inputs as origionally posted:
(New Tire Dia/ Original Tire Dia) x Current Gear Ratio = New Comparitive Gear Ratio
(New Tire Dia/ Original Tire Dia) x Current Gear Ratio = New Comparitive Gear Ratio
Thread Starter
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Postmaster
Joined: Jan 2005
Posts: 2,753
Likes: 3
From: Phenix City, AL
Let me see if I can relate this to beer, that will clear things up a little. Hang on, I'm a little rusty at Drunkanese.
Old beers - 12 oz
New beer - 16 oz
Beer ball - Holds 3.73 OLD beers
(Old Beer / New Beer) x Beer ball = Number of NEW Beers in Beer Ball
(12 / 16) x 3.73 = 2.79 NEW Beers compared to 3.73 OLD Beers
Soooooo, In order to make my beer ball hold 3.73 NEW beers i have to i hold how many old beers?
(12 / 16) x 4.10 = 3.075 NEW beers (still not good enough)
(12 / 16) x 4.30 = 3.225 NEW Beers (still not happy with that)
(12 / 16) x 4.56 = 3.42 NEW Beers (gettin a little happier with my beer ball)
(12 / 16) x 4.88 = 3.66 NEW Beers (this is more to my liking)
(12 / 16) x 5.13 = 3.84 NEW Beers (Oooops, overfilled the beer ball)
So, in order to hold the same number of NEW beers I have to make my beer ball hold 4.88 OLD beers.
Does that make it clearer
Old beers - 12 oz
New beer - 16 oz
Beer ball - Holds 3.73 OLD beers
(Old Beer / New Beer) x Beer ball = Number of NEW Beers in Beer Ball
(12 / 16) x 3.73 = 2.79 NEW Beers compared to 3.73 OLD Beers
Soooooo, In order to make my beer ball hold 3.73 NEW beers i have to i hold how many old beers?
(12 / 16) x 4.10 = 3.075 NEW beers (still not good enough)
(12 / 16) x 4.30 = 3.225 NEW Beers (still not happy with that)
(12 / 16) x 4.56 = 3.42 NEW Beers (gettin a little happier with my beer ball)
(12 / 16) x 4.88 = 3.66 NEW Beers (this is more to my liking)
(12 / 16) x 5.13 = 3.84 NEW Beers (Oooops, overfilled the beer ball)
So, in order to hold the same number of NEW beers I have to make my beer ball hold 4.88 OLD beers.
Does that make it clearer
Mountain Pass
Joined: Jan 2005
Posts: 176
Likes: 1
From: Southeast Texas
This site will factor just about any ratio change you choose to ponder. Check it out http://gs.tolan-hoechst.com/tirecalc.htm