Larger Tires And Re-Gearing - Do It Yourself Calculation
Thread Starter
JK Newbie
Joined: Oct 2006
Posts: 79
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Larger Tires And Re-Gearing - Do It Yourself Calculation
Many folks are interested in running larger tires and different gears,
and they want to know how any given combination will work.
Changing rear-end ratios and changing tires is just like shifting gears.
Getting bigger tires is like upshifting (less engine RPM at any given vehicle
speed). Getting a higher rear-end ratio (changing from 4.10 to 4.56) is like
downshifting (more engine RPM at any given vehicle speed).
Here is a quickie way to figure out how things will change.
If you want bigger tires, divide your old tire diameter by your new tire
diameter. This tells you what your NEW RPM will be.
FOR EXAMPLE, if you want to switch from 32" tires to 35" tires, you
divide 32/35 = 0.9143. So, your NEW RPM = 0.9143 times your OLD RPM.
If you want to re-gear, divide your new ratio by your old ratio. This tells
you what your new RPM will be.
FOR EXAMPLE, if you want to re-gear from 4.10 to 4.56, you divide
4.56/4.10 = 1.1122. So, your NEW RPM = 1.1122 times your OLD RPM.
If you want bigger tires AND new gears, you just multiply the two separate
results together.
FOR EXAMPLE, if you plan to make BOTH of the changes above, you
multiply the two results 0.9143 X 1.1122 = 1.0169. So your NEW RPM
will be 1.0169 times your old RPM.
Doing a change that results in lower NEW RPM will be like driving around in
a higher gear all the time (try leaving a stop sign in 2nd gear).
Doing a change that results in higher NEW RPM will be like driving around in
a lower gear all the time (try cruising the freeway in 5th gear).
That being said, it is ideal to have your NEW RPM a little higher than the
OLD RPM when you get a lift and bigger tires. It is just like driving in a
slightly lower gear to help your rig pull the added weight and drag of the
lift and big tires.
Andrew
and they want to know how any given combination will work.
Changing rear-end ratios and changing tires is just like shifting gears.
Getting bigger tires is like upshifting (less engine RPM at any given vehicle
speed). Getting a higher rear-end ratio (changing from 4.10 to 4.56) is like
downshifting (more engine RPM at any given vehicle speed).
Here is a quickie way to figure out how things will change.
If you want bigger tires, divide your old tire diameter by your new tire
diameter. This tells you what your NEW RPM will be.
FOR EXAMPLE, if you want to switch from 32" tires to 35" tires, you
divide 32/35 = 0.9143. So, your NEW RPM = 0.9143 times your OLD RPM.
If you want to re-gear, divide your new ratio by your old ratio. This tells
you what your new RPM will be.
FOR EXAMPLE, if you want to re-gear from 4.10 to 4.56, you divide
4.56/4.10 = 1.1122. So, your NEW RPM = 1.1122 times your OLD RPM.
If you want bigger tires AND new gears, you just multiply the two separate
results together.
FOR EXAMPLE, if you plan to make BOTH of the changes above, you
multiply the two results 0.9143 X 1.1122 = 1.0169. So your NEW RPM
will be 1.0169 times your old RPM.
Doing a change that results in lower NEW RPM will be like driving around in
a higher gear all the time (try leaving a stop sign in 2nd gear).
Doing a change that results in higher NEW RPM will be like driving around in
a lower gear all the time (try cruising the freeway in 5th gear).
That being said, it is ideal to have your NEW RPM a little higher than the
OLD RPM when you get a lift and bigger tires. It is just like driving in a
slightly lower gear to help your rig pull the added weight and drag of the
lift and big tires.
Andrew
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Good info, Andre, thanks.
Now how does that equate to incorrect SPEEDOMETER & ODOMETER readings when someone on stock 4.10 rear axle replaces the JK's stock 32 inch tires with something bigger (35 inches) or smaller (29 inches)?
Obviously, that would effectively decrease or increase the respective RPMs right there (I estimate by 11% for a 3 inch change, and a whopping 20% for a 5 inch change), but how would the speedometer and odometer readings be affected? I gues I am wondering where those readings are being picked up from in the drive line before being routed to the instruments. Given that so many potential JK users would be altering lift, did the JK engineers have the ingenuity to pick up road speed at some benign spot where these real world variations would be accounted for so that the speedo readings were good readings? Otherwise, 20% is a significant number and a speeding ticket awaiting - when the speedometer reads 55 mph and the car is actually doing 67 mph!
Now how does that equate to incorrect SPEEDOMETER & ODOMETER readings when someone on stock 4.10 rear axle replaces the JK's stock 32 inch tires with something bigger (35 inches) or smaller (29 inches)?
Obviously, that would effectively decrease or increase the respective RPMs right there (I estimate by 11% for a 3 inch change, and a whopping 20% for a 5 inch change), but how would the speedometer and odometer readings be affected? I gues I am wondering where those readings are being picked up from in the drive line before being routed to the instruments. Given that so many potential JK users would be altering lift, did the JK engineers have the ingenuity to pick up road speed at some benign spot where these real world variations would be accounted for so that the speedo readings were good readings? Otherwise, 20% is a significant number and a speeding ticket awaiting - when the speedometer reads 55 mph and the car is actually doing 67 mph!
Thread Starter
JK Newbie
Joined: Oct 2006
Posts: 79
Likes: 0
tires size has an effect. If the pickup is at the trasfer case, then both the
rear-end gears and the tires have an effect.
I wonder where the pickup is?
Andrew
