Gear ratio question
Thread Starter
JK Newbie
Joined: Dec 2016
Posts: 4
Likes: 0
From: Villanova
Gear ratio question
New guy here just purchased my 07 sahara about a month ago. The build sheet says my gear ratio is 3.21 but I think the previous owner swapped out the stock gears. I bought it with 33s, In 4th gear at 60mph rpm is around 2700-2800. How can I calculate my gear ratio?
JK Newbie
Joined: Jun 2015
Posts: 63
Likes: 0
From: Fort Benning
Open the Diff. Cover and count teeth!!
JK Jedi
Joined: Oct 2010
Posts: 6,222
Likes: 376
From: Southwest Reno, NV
Jack the rear wheels off the ground, put it in neutral and turn the driveshaft by hand and count how many times it takes to turn the tires 1 turn.
4 speed automatic right? 60 mph and that RPM should have you at a 5.38 gears.
4 speed automatic right? 60 mph and that RPM should have you at a 5.38 gears.
Last edited by TheDirtman; Dec 11, 2016 at 04:53 PM.
JK Freak
Joined: Apr 2014
Posts: 505
Likes: 4
From: Anaheim, ca
Don't do that!
Raise the tires off the ground & count the driveshaft revolutions while rotating the tire.
Turn the tire 2 complete rotations if your other tire remains stationary and does not rotate. Turn the tire 1 complete rotation if the other tire turns in the same direction.
The number of times the driveshaft rotates equals the gear ratio.
If the driveshaft rotates about 3x, the gear ratio is 3.07. If it rotates 3 3/4 turns, the gear ratio is 3.73. You can mark the driveshaft with a paint pen to help you count.
Raise the tires off the ground & count the driveshaft revolutions while rotating the tire.
Turn the tire 2 complete rotations if your other tire remains stationary and does not rotate. Turn the tire 1 complete rotation if the other tire turns in the same direction.
The number of times the driveshaft rotates equals the gear ratio.
If the driveshaft rotates about 3x, the gear ratio is 3.07. If it rotates 3 3/4 turns, the gear ratio is 3.73. You can mark the driveshaft with a paint pen to help you count.
Thread Starter
JK Newbie
Joined: Dec 2016
Posts: 4
Likes: 0
From: Villanova
sorry 6 speed manual, just going off some charts I found it looks like 4.56 but I wasn't sure how accurate they are
I'll try to tire rotation trick
I'll try to tire rotation trick
Last edited by GregFarz78; Dec 11, 2016 at 05:33 PM.
JK Jedi Master
Joined: Jan 2008
Posts: 11,463
Likes: 165
From: Austin <--> Colorado Springs
If you are set on using rpm instead of getting your hands dirty, open the FAQ's stuck right to the top of this Modified area. Scroll down to the Gears/Drivetrain section and look at those charts. They were designed specifically for our JK's, for each engine/transmission combo.
Do not use the tire mfg's spec for the tire height, break out a tape measure and get an accurate number!! Then jump on the freeway and see what your 6th gear (not 4th) rpms are at 70mph.
Do not use the tire mfg's spec for the tire height, break out a tape measure and get an accurate number!! Then jump on the freeway and see what your 6th gear (not 4th) rpms are at 70mph.
JK Jedi Master
Joined: Feb 2008
Posts: 12,904
Likes: 185
From: Maine
Chock the rear wheels.
Jack one front tire off the ground.
Put a chalk mark on the tire and the drive shaft.
Rotate the tire by hand 2 complete revolutions while counting the drive shaft turns.
The number will be your gear ratio.
A little less than 3¼ turns will be 3.21 gears.
About 3¾ turns will be 3.73 gears.
A little over 4 turns will be 4.10 gears.
About 4½ turns will be 4.56 gears
A little less than 5 turns will be 4.88 gears
Jack one front tire off the ground.
Put a chalk mark on the tire and the drive shaft.
Rotate the tire by hand 2 complete revolutions while counting the drive shaft turns.
The number will be your gear ratio.
A little less than 3¼ turns will be 3.21 gears.
About 3¾ turns will be 3.73 gears.
A little over 4 turns will be 4.10 gears.
About 4½ turns will be 4.56 gears
A little less than 5 turns will be 4.88 gears
