Gear Ratio & Tire Size Chart
03-12-2012, 06:05 AM
#61
JK Newbie
Join Date: Jan 2012
Location: Watertown, New York
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
My question is if you are running at 3,000 rpms on the freewy what is your gas mileage? witch would be smarter if you have 35's and you drive alot on the hwy? would it be 5.13 or 5.38 with the six speed manual trany.
That chart is very misleading because it is based on a manual transmission gear ratio of 1.0. It leads people to under gear their rigs.
Quote: These calculations are assuming a manual transmission with a 1:1 ratio
Automatics have a 4th gear (overdrive) ratio of 0.69.
6 speeds have a 6th gear ratio of 0.84.
The experienced jeepers (who have driven different setups with different gear ratios) nearly always give unanimous answers:
With a non-supercharged engine:
An automatic on 35s or larger should run 5.38s if they drive mountain passes, live at higher elevations, tow anything, or ever plan to upgrade to 37s--especially if they drive a 4 door.
An automatic on 35s that lives and wheels at low elevations, doesn't drive steep grades, or doesn't tow anything should run 5.13s.
A 6 speed on 37s should run 5.13s, and maybe 5.38s if they don't spend much time over 65 mph.
A 6 speed on 35s that is either a 4 dr, or drives at high elevations, tows, or drives steep grades should run 5.13s.
A 6 speed on 35s that is a 2 door and will never upgrade to 37s, or a 4 door that lives and wheels at low elevations and almost never tows should run 4.88s.
There is very minimal debate about this among guys who have had the experience of driving different setups with different gear ratios. They guys who suggest otherwise usually have never driven or experienced different gear ratios.
The shops discourage the 5.13 and 5.38 gears because they don't understand our engines only produce 64 rear wheel horsepower 169 torque at 2000 rpm, at 2500 rpm they produce 88 horsepower and 184 torque, and at 3000 rpm they produce 107 horsepower and 188 ft lbs of torque.
They don't understand that automatics have a 4th gear ratio of 0.69. These mini-van engines need higher rpms to produce enough horsepower and torque to turn heavy 35"+ tires--especially up steep freeway grades or at higher elevations.
Okay, rant over. It is just that every time I see that table on quadratec, 4wd, and other websites, it reminds me of all the folks who wasted $1000-$1500 to gear to the wrong gear ratio--then they believe they need to spend more money on engine upgrades (superchargers, cold air intakes, headers, etc.) because they are not satisfied with the weak performance of our engines at rpms under 2500-3000. They don't even get that the table is based on a 1.0 transmission gear ratio for a manual transmission. They could have almost doubled their rear wheel horsepower if they had just properly geared their rigs to run 3k rpm at freeway speeds.
Quote: These calculations are assuming a manual transmission with a 1:1 ratio
Automatics have a 4th gear (overdrive) ratio of 0.69.
6 speeds have a 6th gear ratio of 0.84.
The experienced jeepers (who have driven different setups with different gear ratios) nearly always give unanimous answers:
With a non-supercharged engine:
An automatic on 35s or larger should run 5.38s if they drive mountain passes, live at higher elevations, tow anything, or ever plan to upgrade to 37s--especially if they drive a 4 door.
An automatic on 35s that lives and wheels at low elevations, doesn't drive steep grades, or doesn't tow anything should run 5.13s.
A 6 speed on 37s should run 5.13s, and maybe 5.38s if they don't spend much time over 65 mph.
A 6 speed on 35s that is either a 4 dr, or drives at high elevations, tows, or drives steep grades should run 5.13s.
A 6 speed on 35s that is a 2 door and will never upgrade to 37s, or a 4 door that lives and wheels at low elevations and almost never tows should run 4.88s.
There is very minimal debate about this among guys who have had the experience of driving different setups with different gear ratios. They guys who suggest otherwise usually have never driven or experienced different gear ratios.
The shops discourage the 5.13 and 5.38 gears because they don't understand our engines only produce 64 rear wheel horsepower 169 torque at 2000 rpm, at 2500 rpm they produce 88 horsepower and 184 torque, and at 3000 rpm they produce 107 horsepower and 188 ft lbs of torque.
They don't understand that automatics have a 4th gear ratio of 0.69. These mini-van engines need higher rpms to produce enough horsepower and torque to turn heavy 35"+ tires--especially up steep freeway grades or at higher elevations.
Okay, rant over. It is just that every time I see that table on quadratec, 4wd, and other websites, it reminds me of all the folks who wasted $1000-$1500 to gear to the wrong gear ratio--then they believe they need to spend more money on engine upgrades (superchargers, cold air intakes, headers, etc.) because they are not satisfied with the weak performance of our engines at rpms under 2500-3000. They don't even get that the table is based on a 1.0 transmission gear ratio for a manual transmission. They could have almost doubled their rear wheel horsepower if they had just properly geared their rigs to run 3k rpm at freeway speeds.
03-13-2012, 03:50 AM
#62
JK Junkie
Join Date: Jul 2008
Location: Lakewood, OH
Posts: 3,017
Likes: 0
Received 0 Likes
on
0 Posts
You need to look at a lot of things. The speed you drive, the terrain you drive over and auxiliary needs such as towing. 5.13s or 5.38s are pretty steep for a 6 speed manual and the 3.8L. Rolling hills and higher speeds 4.88s would be the call. Lower cruising speeds under 65 mph and flat ground 4.56s would work. Cruising rpms above 2700 are going to rob efficiency, and cruising rpms below 2300 start placing a need on reducing velocity.
Another question to ask yourself is are you buying gears for performance or efficiency. At 2300 rpms you are going to be running pretty efficiently, at 2700 rpms you are going to have nominally less efficiency but reduce the frequency of downshifts. It's definitely a mixed bag but with a 6 speed you should be able to find what gear is working best for you at cruise speeds before you do a gear swap, and do the math to find out which gear set makes the most sense.
3.21s 5th gear = 3.82 suggesting 3.73/4.10 gears
3.21s 4th gear = 4.78 suggesting 4.56/4.88 gears
3.73s 5th gear = 4.44 suggesting 4.10/4.56/4.88 gears
3.73s 4th gear = 5.55 suggesting 5.13/5.38 gears
Another question to ask yourself is are you buying gears for performance or efficiency. At 2300 rpms you are going to be running pretty efficiently, at 2700 rpms you are going to have nominally less efficiency but reduce the frequency of downshifts. It's definitely a mixed bag but with a 6 speed you should be able to find what gear is working best for you at cruise speeds before you do a gear swap, and do the math to find out which gear set makes the most sense.
3.21s 5th gear = 3.82 suggesting 3.73/4.10 gears
3.21s 4th gear = 4.78 suggesting 4.56/4.88 gears
3.73s 5th gear = 4.44 suggesting 4.10/4.56/4.88 gears
3.73s 4th gear = 5.55 suggesting 5.13/5.38 gears
06-11-2012, 05:27 PM
#63
JK Jedi
Just traded my jacked up ram for a 2010 JK that has a 4" Rancho long arm suspension lift 18x9 wheels and 37X12.50R18 BFG Mud Terrain KM2 Tires... I had 315/75R16 on my Ram and ran 4:56 gears and was fine.... But this jk has the stock 3:73 gears and they struggle with those tires.... Should I go with 5:13s or 5:38s?? I live in south jersey mostly pineland trails... Some mud but mostly soft sand some steep grades.... But I also use this everyday to get around and I need to be able to drive on the hiway to work.... 4:56 was the highest ive ever had and just want some opinions from ppl who might be running that setup
06-11-2012, 05:45 PM
#64
JK Freak
Join Date: Jan 2011
Location: Poway, California
Posts: 761
Likes: 0
Received 0 Likes
on
0 Posts
Just traded my jacked up ram for a 2010 JK that has a 4" Rancho long arm suspension lift 18x9 wheels and 37X12.50R18 BFG Mud Terrain KM2 Tires... I had 315/75R16 on my Ram and ran 4:56 gears and was fine.... But this jk has the stock 3:73 gears and they struggle with those tires.... Should I go with 5:13s or 5:38s?? I live in south jersey mostly pineland trails... Some mud but mostly soft sand some steep grades.... But I also use this everyday to get around and I need to be able to drive on the hiway to work.... 4:56 was the highest ive ever had and just want some opinions from ppl who might be running that setup
06-12-2012, 05:04 PM
#67
JK Jedi
10-01-2012, 05:31 AM
#68
JK Newbie
Join Date: Sep 2012
Location: Dallas, TX
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
I just had 5.38 installed on my 2010 Rubicon with 35". I don't know all the math or anything, but I can tell you it has made a world of difference in driving. I have been getting right at 15.5 mpg keeping it running at 70 mph. It is amazing how much you save in gas not constantly shifting out of overdrive to try and maintain speed. Normal driving I do around 65 mph and that gets a little over 16 mpg.