Planning on regearing? You may want to read
04-26-2012, 07:25 PM
#31
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Ok, let's all do the math... 
Maybe I'm full of manure (and please tell me why I am) but assuming you only want to keep the same RPM at highway speed (no torque increase)...
If I go from 32" to 35" tires my TIRE revolutions per mile decrease from 649 to 598 (from tire manufacturer specs). That means to keep the same ENGINE RPMs I need to increase my gear ratio by 8.5% (649/598 = 1.085284). So:
Starting with 3.21 gears, changing to 3.73 gears over corrects by 7% [i.e.: your engine RPMs will be 7% higher... (598/649)*(3.73/3.21) = 1.07]
Starting with 3.73 gears with 35" tires you will already be way ahead of stock 32" tires with 3.21 gears.
To keep the exact same RPMs as 3.73 gears while going from 32" to 35" tires you would need 4.05 (imaginary) gears! (598/649)*(4.05/3.73) = 1.00
More mental gymnastics:
If you start with 32" tires and switch to 35" tires and...
... change from 3.21 to 4.88 gears your RPMs will increase by (598/649)*(4.88/3.21) = 1.40 That's a 40% INCREASE in engine RPMs!
... change from 3.73 to 5.13 gears your RPMs will increase by (598/649)*(5.13/3.73) = 1.27 That's a 27% increase.
... change from 3.73 to 4.56 gears your RPMs will increase by (598/649)*(4.56/3.73) = 1.13 or 13%.
... change from 3.73 to 4.1 gears your RPMs will increase by (598/649)*(4.1/3.73) = 1.01 or 1%.
These numbers agree with the "gear chart" on this site when comparing RPM values.
But RPMs don't tell the whole story. Because bigger tires have more wind resistance and rolling resistance it will take more power (gas) to move your Jeep. A higher gearing (higher RPM) will give you more torque (giddy up) for faster acceleration but you will still lose gas mileage and top end.
Just a thought. Your mileage may vary. Void where prohibited. Professional driver on a closed track. Don't try this at home.
Maybe I'm full of manure (and please tell me why I am) but assuming you only want to keep the same RPM at highway speed (no torque increase)...
If I go from 32" to 35" tires my TIRE revolutions per mile decrease from 649 to 598 (from tire manufacturer specs). That means to keep the same ENGINE RPMs I need to increase my gear ratio by 8.5% (649/598 = 1.085284). So:
Starting with 3.21 gears, changing to 3.73 gears over corrects by 7% [i.e.: your engine RPMs will be 7% higher... (598/649)*(3.73/3.21) = 1.07]
Starting with 3.73 gears with 35" tires you will already be way ahead of stock 32" tires with 3.21 gears.
To keep the exact same RPMs as 3.73 gears while going from 32" to 35" tires you would need 4.05 (imaginary) gears! (598/649)*(4.05/3.73) = 1.00
More mental gymnastics:
If you start with 32" tires and switch to 35" tires and...
... change from 3.21 to 4.88 gears your RPMs will increase by (598/649)*(4.88/3.21) = 1.40 That's a 40% INCREASE in engine RPMs!
... change from 3.73 to 5.13 gears your RPMs will increase by (598/649)*(5.13/3.73) = 1.27 That's a 27% increase.
... change from 3.73 to 4.56 gears your RPMs will increase by (598/649)*(4.56/3.73) = 1.13 or 13%.
... change from 3.73 to 4.1 gears your RPMs will increase by (598/649)*(4.1/3.73) = 1.01 or 1%.
These numbers agree with the "gear chart" on this site when comparing RPM values.
But RPMs don't tell the whole story. Because bigger tires have more wind resistance and rolling resistance it will take more power (gas) to move your Jeep. A higher gearing (higher RPM) will give you more torque (giddy up) for faster acceleration but you will still lose gas mileage and top end.
Just a thought. Your mileage may vary. Void where prohibited. Professional driver on a closed track. Don't try this at home.
04-26-2012, 08:01 PM
#32
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Originally Posted by Sahara Lee
Ok, let's all do the math...
Maybe I'm full of manure (and please tell me why I am) but assuming you only want to keep the same RPM at highway speed (no torque increase)...
If I go from 32" to 35" tires my TIRE revolutions per mile decrease from 649 to 598 (from tire manufacturer specs). That means to keep the same ENGINE RPMs I need to increase my gear ratio by 8.5% (649/598 = 1.085284). So:
Starting with 3.21 gears, changing to 3.73 gears over corrects by 7% [i.e.: your engine RPMs will be 7% higher... (598/649)*(3.73/3.21) = 1.07]
Starting with 3.73 gears with 35" tires you will already be way ahead of stock 32" tires with 3.21 gears.
To keep the exact same RPMs as 3.73 gears while going from 32" to 35" tires you would need 4.05 (imaginary) gears! (598/649)*(4.05/3.73) = 1.00
More mental gymnastics:
If you start with 32" tires and switch to 35" tires and...
... change from 3.21 to 4.88 gears your RPMs will increase by (598/649)*(4.88/3.21) = 1.40 That's a 40% INCREASE in engine RPMs!
... change from 3.73 to 5.13 gears your RPMs will increase by (598/649)*(5.13/3.73) = 1.27 That's a 27% increase.
... change from 3.73 to 4.56 gears your RPMs will increase by (598/649)*(4.56/3.73) = 1.13 or 13%.
... change from 3.73 to 4.1 gears your RPMs will increase by (598/649)*(4.1/3.73) = 1.01 or 1%.
These numbers agree with the "gear chart" on this site when comparing RPM values.
But RPMs don't tell the whole story. Because bigger tires have more wind resistance and rolling resistance it will take more power (gas) to move your Jeep. A higher gearing (higher RPM) will give you more torque (giddy up) for faster acceleration but you will still lose gas mileage and top end.
Just a thought. Your mileage may vary. Void where prohibited. Professional driver on a closed track. Don't try this at home.
Maybe I'm full of manure (and please tell me why I am) but assuming you only want to keep the same RPM at highway speed (no torque increase)...
If I go from 32" to 35" tires my TIRE revolutions per mile decrease from 649 to 598 (from tire manufacturer specs). That means to keep the same ENGINE RPMs I need to increase my gear ratio by 8.5% (649/598 = 1.085284). So:
Starting with 3.21 gears, changing to 3.73 gears over corrects by 7% [i.e.: your engine RPMs will be 7% higher... (598/649)*(3.73/3.21) = 1.07]
Starting with 3.73 gears with 35" tires you will already be way ahead of stock 32" tires with 3.21 gears.
To keep the exact same RPMs as 3.73 gears while going from 32" to 35" tires you would need 4.05 (imaginary) gears! (598/649)*(4.05/3.73) = 1.00
More mental gymnastics:
If you start with 32" tires and switch to 35" tires and...
... change from 3.21 to 4.88 gears your RPMs will increase by (598/649)*(4.88/3.21) = 1.40 That's a 40% INCREASE in engine RPMs!
... change from 3.73 to 5.13 gears your RPMs will increase by (598/649)*(5.13/3.73) = 1.27 That's a 27% increase.
... change from 3.73 to 4.56 gears your RPMs will increase by (598/649)*(4.56/3.73) = 1.13 or 13%.
... change from 3.73 to 4.1 gears your RPMs will increase by (598/649)*(4.1/3.73) = 1.01 or 1%.
These numbers agree with the "gear chart" on this site when comparing RPM values.
But RPMs don't tell the whole story. Because bigger tires have more wind resistance and rolling resistance it will take more power (gas) to move your Jeep. A higher gearing (higher RPM) will give you more torque (giddy up) for faster acceleration but you will still lose gas mileage and top end.
Just a thought. Your mileage may vary. Void where prohibited. Professional driver on a closed track. Don't try this at home.
04-26-2012, 08:20 PM
#33
JK Enthusiast
I personally think the tires you plan on running should play a part in your gear selection. When you are running big and really heavy tires, taller gears dont seem quite as bad...
04-26-2012, 08:24 PM
#34
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Fixed it for you. My first 2011 Rubicon was a 6-speed and it was like driving a dump truck. I'm sure that most people jump from 3.73 or 4.10 to 4.88 and it doesn't seem as drastic.
Last edited by JamesWyatt; 04-26-2012 at 08:26 PM.
04-27-2012, 02:15 AM
#35
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BFG tires run notoriously smaller than their advertised claims. The 315 70R17s are probably running closer to 33" than 35". If you are running a 9+" wheel width they might not even measure 33".
I don't see any mention of a ProCal. To have any reliability in your fuel economy you need to have a calibrated speedometer which in turn will give you a calibrated odometer. Fuel mileage numbers won't mean anything without one, especially if you are using the dash computer.
4.88s are a performance gear set for people living in the flatlands and running 35s with a 6 speed. If your tires are running 5 or 6% shorter, 4.88s are a steep gear for your local driving although they will have great off road performance.
Suggestions for improving your situation.
Buy a ProCal. It's impossible to know where you really stand without having a calibrated vehicle.
Measure your tires from the ground to the tread shoulder. That will give you a real tire height rolling diameter, not the advertised height.
If you are running 9" wide wheels, consider moving to 8" wheels. Every inch narrower wheel width will give you roughly an additional quarter inch tire height.
Inflation rate. Keeping your tires aired up to 30psi or modestly higher might sacrifice some comfort, but will improve fuel mileage, still have good wear and make a slight difference in rolling height.
Hope that helps.
I don't see any mention of a ProCal. To have any reliability in your fuel economy you need to have a calibrated speedometer which in turn will give you a calibrated odometer. Fuel mileage numbers won't mean anything without one, especially if you are using the dash computer.
4.88s are a performance gear set for people living in the flatlands and running 35s with a 6 speed. If your tires are running 5 or 6% shorter, 4.88s are a steep gear for your local driving although they will have great off road performance.
Suggestions for improving your situation.
Buy a ProCal. It's impossible to know where you really stand without having a calibrated vehicle.
Measure your tires from the ground to the tread shoulder. That will give you a real tire height rolling diameter, not the advertised height.
If you are running 9" wide wheels, consider moving to 8" wheels. Every inch narrower wheel width will give you roughly an additional quarter inch tire height.
Inflation rate. Keeping your tires aired up to 30psi or modestly higher might sacrifice some comfort, but will improve fuel mileage, still have good wear and make a slight difference in rolling height.
Hope that helps.
04-27-2012, 04:25 AM
#36
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I always prefered this chart, as it breaks it down for manuals and auto's. For my uses and since i drive a manual i think 4:56 is the way to go, but yes like you the shop keeps insisting i put in 5:13's
04-27-2012, 06:04 AM
#37
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I have a buddy who has 5.38s on his 6 spd with 38" tires and is quite happy. I have 5.13s on my auto with 35s and am very happy. But we do hard core wheeling too.
People try to oversimplify this but it's not a one size fits all - whether the size is 4.56, 4.88, or 5.13. Setups are different and needs are different.
I did a lot of discussion with people in the hobby, mechanics who specialize in 4x4s, and even a call to Randy's to discuss setup before making a decision.
People try to oversimplify this but it's not a one size fits all - whether the size is 4.56, 4.88, or 5.13. Setups are different and needs are different.
I did a lot of discussion with people in the hobby, mechanics who specialize in 4x4s, and even a call to Randy's to discuss setup before making a decision.
04-27-2012, 07:30 AM
#38
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Sadly Ive had that chart on my phone for a year now and I even showed it to the guy at the shop he said I should still do 4.88s, I wish I knew that chart was for tire size while on vehicle as a previous poster stated BFGs do run small and 4.56 would have been better maybe even 4.10.
04-27-2012, 07:32 AM
#39
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BFG tires run notoriously smaller than their advertised claims. The 315 70R17s are probably running closer to 33" than 35". If you are running a 9+" wheel width they might not even measure 33".
I don't see any mention of a ProCal. To have any reliability in your fuel economy you need to have a calibrated speedometer which in turn will give you a calibrated odometer. Fuel mileage numbers won't mean anything without one, especially if you are using the dash computer.
4.88s are a performance gear set for people living in the flatlands and running 35s with a 6 speed. If your tires are running 5 or 6% shorter, 4.88s are a steep gear for your local driving although they will have great off road performance.
Suggestions for improving your situation.
Buy a ProCal. It's impossible to know where you really stand without having a calibrated vehicle.
Measure your tires from the ground to the tread shoulder. That will give you a real tire height rolling diameter, not the advertised height.
If you are running 9" wide wheels, consider moving to 8" wheels. Every inch narrower wheel width will give you roughly an additional quarter inch tire height.
Inflation rate. Keeping your tires aired up to 30psi or modestly higher might sacrifice some comfort, but will improve fuel mileage, still have good wear and make a slight difference in rolling height.
Hope that helps.
I don't see any mention of a ProCal. To have any reliability in your fuel economy you need to have a calibrated speedometer which in turn will give you a calibrated odometer. Fuel mileage numbers won't mean anything without one, especially if you are using the dash computer.
4.88s are a performance gear set for people living in the flatlands and running 35s with a 6 speed. If your tires are running 5 or 6% shorter, 4.88s are a steep gear for your local driving although they will have great off road performance.
Suggestions for improving your situation.
Buy a ProCal. It's impossible to know where you really stand without having a calibrated vehicle.
Measure your tires from the ground to the tread shoulder. That will give you a real tire height rolling diameter, not the advertised height.
If you are running 9" wide wheels, consider moving to 8" wheels. Every inch narrower wheel width will give you roughly an additional quarter inch tire height.
Inflation rate. Keeping your tires aired up to 30psi or modestly higher might sacrifice some comfort, but will improve fuel mileage, still have good wear and make a slight difference in rolling height.
Hope that helps.
Well the issue really isn't fuel economy it is the fact that the engine is now working harder than it needs to, my speedometer is calibrated anyway. I'm just concerned I'm putting extra work on the engine when I don't need to. I can shift gears 2 sometimes 3 times by the time i get through a red light intersection, that's my concern.
04-27-2012, 10:17 AM
#40
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Well the issue really isn't fuel economy it is the fact that the engine is now working harder than it needs to, my speedometer is calibrated anyway. I'm just concerned I'm putting extra work on the engine when I don't need to. I can shift gears 2 sometimes 3 times by the time i get through a red light intersection, that's my concern.
Engines don't work harder with taller gears as there's less power needed per stroke, they simply just spin more. Engines have an efficiency curve, they don't perform efficiently at the low end of the rpm range nor do they perform efficiently as you approach peak horsepower.
You should throw out some rpm numbers at your cruise speeds so there's a better feeling for your situation.
Last edited by JPop; 04-27-2012 at 10:55 AM.