Addressing your differential swap RPM concerns. - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums

6bangerBOOM · 11-17-2013, 02:33 AM

As I've said in a few posts I'm about to do a gear swap to a 1LE differential housing (which is really just an SS housing, but it says 1LE on my invoice damnit!) with 3.91 gear ratio. Many of you, myself included, are concerned about what this will do to your cruising RPM's. I'm especially anxious as my family lives about 800 miles away from where I live/work.

There are many calculators out there that can be located with a simple google search, but you're a DIY'er aren't ya? So, here is the formula with a meaningful breakdown to follow. This will allow you to understand how changes to your vehicle's gear reduction at either the transmission or the differential will affect your drivetrain's performance. This formula is for figuring out what your MPH will be at a given RPM (and vice versa), and has 5 steps as follows:

Note: When I say ratio I really mean the first number of the ratio. So when I say divide by 3.91 gear ratio I mean 3.91 and not 3.91:1.

1) Engine RPM/ Transmission gear ratio= Driveshaft RPM
2) Driveshaft RPM/Rear Differential Gear ratio= Axle RPM
3) Axle RPM x Wheel Circumference= Distance traveled per minute in ins.
4) Distance per minute in ins./ 63360= Distance per minute in miles
5) Distance travelled per minute in miles x 60= Miles per hour!
You can flip flop this process (go backwards and change div. to mult. and mult. to div.) to determine RPM's at miles per hour.

Check the wiki section of this site to determine your transmission gear ratios/diff ratio. Assuming 2LS status you are as follows:

1st gear: 4.07
2nd gear: 2.37
3rd gear: 1.55
4th gear: 1.16
5th gear: 0.85
6th gear: 0.67
Rear diff/Final drive ratio: 2.92

Explanation of formula steps:
Step #1) Engine rpm divided by the gear ratio of the transmission gear you are in will yield the rpms at your drive shaft.

Step #2) The RPM of your driveshaft divided by the rear diff gear ratio gives the RPM at the axles.

Step #3) Axle RPM multiplied by wheel circumfernece yields the distance your wheel will travel per minute in inches. Your wheel circumference is the diameter of your wheel multiplied by Pi (3.14yaddayaddayadda). If you don't have a graphing calculator handy multiplying your wheel diameter by 3.14 WILL get you close enough!

Step #4) Because we calculated your wheel circumference in inches (it would be weird to calculate wheel circumference in miles I think, though technically not impossible) we must now convert to miles. This can be handled with a speedy division by 63360 (1mile = 63360 inches). This yields the distance your wheel will travel per minute in miles. Lastly...

Step #5) To convert this miles per minute to a proper MPH figure we need to multiply by 60. You are now looking at the MPH your car can achieve at a given engine rpm with given gear ratios. Did you have fun?!?!

Lets do an example before I press submit.

I'm at 2000 rpms in my stock 2LS and I'm in 6th gear (0.67). This means my drive shaft will be turning at 2985 RPM (I'm leaving off the decimal figures here but still using them in my calculations). With this RPM at the drive shaft, and with a stock rear diff ratio of 2.92, my axle RPM will be 1022. My wheels/tires are approx 28.5 inches tall so (28.5 x Pi) my wheel circumference is 89.54 inches. So, now I take my axle RPMs (1022) and multiply them by my wheel circumference (89.54) and this shows that I will be travelling 91,535 IPM (inches per minute). Cleaning this figure up with the last two steps will yield the Prius pwning figure of 86.68 MPH at 2000rpms.

When you start to plug in the 3.55's, 3.73's, and 3.91's you may cringe, but dont forget the appeal of the low range neck workout you'll be getting with lowered gears (not to mention a sore perma-grin area).

If none of this made any damn sense please check out the following:
http://www.bgsoflex.com/rpmmph.html

simt3k · 11-17-2013, 10:58 AM

Could you list all your part numbers for your swap as well? I am interested in this.

6bangerBOOM · 11-17-2013, 03:46 PM

Quote:

Originally Posted by simt3k

Could you list all your part numbers for your swap as well? I am interested in this.

They didn't put the part numbers on the invoice, but here's my part list for this
build.

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11-17-2013, 02:33 AM	#1
6bangerBOOM Drives: 2016 1SS SW Join Date: Jun 2013 Location: N/A Posts: 1,443	Addressing your differential swap RPM concerns. As I've said in a few posts I'm about to do a gear swap to a 1LE differential housing (which is really just an SS housing, but it says 1LE on my invoice damnit!) with 3.91 gear ratio. Many of you, myself included, are concerned about what this will do to your cruising RPM's. I'm especially anxious as my family lives about 800 miles away from where I live/work. There are many calculators out there that can be located with a simple google search, but you're a DIY'er aren't ya? So, here is the formula with a meaningful breakdown to follow. This will allow you to understand how changes to your vehicle's gear reduction at either the transmission or the differential will affect your drivetrain's performance. This formula is for figuring out what your MPH will be at a given RPM (and vice versa), and has 5 steps as follows: Note: When I say ratio I really mean the first number of the ratio. So when I say divide by 3.91 gear ratio I mean 3.91 and not 3.91:1. 1) Engine RPM/ Transmission gear ratio= Driveshaft RPM 2) Driveshaft RPM/Rear Differential Gear ratio= Axle RPM 3) Axle RPM x Wheel Circumference= Distance traveled per minute in ins. 4) Distance per minute in ins./ 63360= Distance per minute in miles 5) Distance travelled per minute in miles x 60= Miles per hour! You can flip flop this process (go backwards and change div. to mult. and mult. to div.) to determine RPM's at miles per hour. Check the wiki section of this site to determine your transmission gear ratios/diff ratio. Assuming 2LS status you are as follows: 1st gear: 4.07 2nd gear: 2.37 3rd gear: 1.55 4th gear: 1.16 5th gear: 0.85 6th gear: 0.67 Rear diff/Final drive ratio: 2.92 Explanation of formula steps: Step #1) Engine rpm divided by the gear ratio of the transmission gear you are in will yield the rpms at your drive shaft. Step #2) The RPM of your driveshaft divided by the rear diff gear ratio gives the RPM at the axles. Step #3) Axle RPM multiplied by wheel circumfernece yields the distance your wheel will travel per minute in inches. Your wheel circumference is the diameter of your wheel multiplied by Pi (3.14yaddayaddayadda). If you don't have a graphing calculator handy multiplying your wheel diameter by 3.14 WILL get you close enough! Step #4) Because we calculated your wheel circumference in inches (it would be weird to calculate wheel circumference in miles I think, though technically not impossible) we must now convert to miles. This can be handled with a speedy division by 63360 (1mile = 63360 inches). This yields the distance your wheel will travel per minute in miles. Lastly... Step #5) To convert this miles per minute to a proper MPH figure we need to multiply by 60. You are now looking at the MPH your car can achieve at a given engine rpm with given gear ratios. Did you have fun?!?! Lets do an example before I press submit. I'm at 2000 rpms in my stock 2LS and I'm in 6th gear (0.67). This means my drive shaft will be turning at 2985 RPM (I'm leaving off the decimal figures here but still using them in my calculations). With this RPM at the drive shaft, and with a stock rear diff ratio of 2.92, my axle RPM will be 1022. My wheels/tires are approx 28.5 inches tall so (28.5 x Pi) my wheel circumference is 89.54 inches. So, now I take my axle RPMs (1022) and multiply them by my wheel circumference (89.54) and this shows that I will be travelling 91,535 IPM (inches per minute). Cleaning this figure up with the last two steps will yield the Prius pwning figure of 86.68 MPH at 2000rpms. When you start to plug in the 3.55's, 3.73's, and 3.91's you may cringe, but dont forget the appeal of the low range neck workout you'll be getting with lowered gears (not to mention a sore perma-grin area). If none of this made any damn sense please check out the following: http://www.bgsoflex.com/rpmmph.html Last edited by 6bangerBOOM; 11-18-2013 at 12:37 AM. Reason: Cain't spel gud

11-17-2013, 10:58 AM	#2
simt3k Drives: 2011 Camaro RS 2LT Join Date: Jul 2012 Location: Edmonton Alberta Canada Posts: 762	Could you list all your part numbers for your swap as well? I am interested in this.