Calculator for choosing your rear axle gear
08-04-2008, 10:26 AM
#1
JK Freak
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Calculator for choosing your rear axle gear
fyi - here's a chart for calculating true RPM/MPH
http://www.angelfire.com/fl/procrastination/rear.html
RPM Gear ratio 4th (auto) Tire MPH
2000 4.10 .69 34.6 73
2000 4.56 .69 34.6 65
2000 4.88 .69 34.6 61
2000 5.13 .69 34.6 58
* 2000 is a good cruising rpm
since 95% of my driving is on highway - 4.10 or 4.56 gears should work, but leaving room and going with a 4.88 to allow for 37" down the road.
http://www.angelfire.com/fl/procrastination/rear.html
RPM Gear ratio 4th (auto) Tire MPH
2000 4.10 .69 34.6 73
2000 4.56 .69 34.6 65
2000 4.88 .69 34.6 61
2000 5.13 .69 34.6 58
* 2000 is a good cruising rpm
since 95% of my driving is on highway - 4.10 or 4.56 gears should work, but leaving room and going with a 4.88 to allow for 37" down the road.
08-04-2008, 10:40 AM
#2
JK Freak
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I must be doing something wrong on that website, because on the way to Corpus last month, all open highway. I was running 3100rpm, and running about 80mph
OK so if I input
3100 rpm
4.10 gear
.69 ratio
35" tire
it says I was going 114mph
OK... so maybe the tires aren't actually 35's. So let's put them at 33.5" for the sake of testing... this results in 109mph.
Something ain't right.
I'm thinking the .69 ratio is incorrect, but I don't know what the ratio actually is for an '07 Unlimited X with auto
OK so if I input
3100 rpm
4.10 gear
.69 ratio
35" tire
it says I was going 114mph
OK... so maybe the tires aren't actually 35's. So let's put them at 33.5" for the sake of testing... this results in 109mph.
Something ain't right.
I'm thinking the .69 ratio is incorrect, but I don't know what the ratio actually is for an '07 Unlimited X with automy calc's were dead on - you must have been flying! either that, or did you have the overdrive turned off? or in 2nd gear?
3100 RPM's are pretty high!
08-04-2008, 10:44 AM
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JK Freak
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speedometer
I must be doing something wrong on that website, because on the way to Corpus last month, all open highway. I was running 3100rpm, and running about 80mph
OK so if I input
3100 rpm
4.10 gear
.69 ratio
35" tire
it says I was going 114mph
OK... so maybe the tires aren't actually 35's. So let's put them at 33.5" for the sake of testing... this results in 109mph.
Something ain't right. I'm thinking the .69 ratio is incorrect, but I don't know what the ratio actually is for an '07 Unlimited X with auto
OK so if I input
3100 rpm
4.10 gear
.69 ratio
35" tire
it says I was going 114mph
OK... so maybe the tires aren't actually 35's. So let's put them at 33.5" for the sake of testing... this results in 109mph.
Something ain't right.
I'm thinking the .69 ratio is incorrect, but I don't know what the ratio actually is for an '07 Unlimited X with autoalso did you have your speedometer recalculated after adding larger tires? where you flying by everyone?
08-04-2008, 11:09 AM
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JK Freak
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08-05-2008, 07:53 AM
#7
JK Freak
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4.88 gears
my usage is 95% daily driver and 5% off-road - if it were reverse, and 95% off-road i would go with 5.13.
using the calculator
you can see where 5.13's will kill you on highway with auto/ .69 final gearing...
@2000 RPM you are only going 58 MPH
the recommended rear gear is actually 4.56...
@2000 RPM you are going 65 MPH = perfect
the 4.88...
@2000 RPM you are only going 61 MPH
the only reason i'm going with the 4.88 vs 4.56 is that i will have the option to go with 37" tires and be perfectly geared for all around performance. won't be the best for gas or best for trails but all around (like stock) settings.
with 4.88 and 37" tires...
@2000 RPM you are only going 65 MPH = perfect
surprisingly, the CAI i just installed actually does make a difference.
my milage went from 20mpg to 16mpg on highway with 35" tires. with the CAI i can actually get 19MPG, plus i can jump on the throttle and seems a little more peppy. so gives you option of power or economy depending on how you drive.