What are your rpm's with 4.88s?
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Posts: n/a
I think its just some general confusion regarding "effects" on the DEVICE that's doing the MEASUREMENT, - vs - the actual change in value.
Essentially, is the change affecting the SPEEDOMETER, or affecting the SPEED?
For example...it seems that the JK uses a Hall Effect speed sensor that reads off of the drive shaft revolutions, to calculate the ground speed, and tone ring sensors on the wheels to calculate wheel spin/ABS/Traction Control, etc...issues.
That means that the SPEEDOMETER is calibrated to interpret a given number of SHAFT RPM as corresponding to a particular SPEED...assuming it knows what gear you are in, your tire size, and your diff ratio.
If you change the diff ratio...say with lower gearing....you raise your ENGINE RPM (Tach reading) for a given real speed...as the engine has to spin faster to make the Jeep go the same speed.
So - the new diff ratio changed the speed the engine ran at, to compensate for the shorter distance each engine revolution sent you...BUT, changing the diff ratio didn't affect the Tach itself....the Tach was just reporting the facts regarding the new corresponding ENGINE RPM.
Same with the speedometer....if the shaft it reads the speed off of, is actually going faster....it thinks the truck is going faster....if the shaft is going faster because the new gears make it HAVE to go faster, to go the same speed...the new gears didn't affect the SPEEDOMETER, they affected the READING of the SPEEDOMETER.
Does this make sense?
If it does make sense, do the same for a larger tire...as the larger tires will make that shaft spin SLOWER, as every tire revolution covers MORE DISTANCE than before.
AND - as you go further for every tire revolution....the engine doesn't have to spin as fast as it used to to go the same speed.
So the tire made the tach SHOW that the engine RPM is lower than before...because it IS slower than before.
AND
Because the tire covers more distance with every tire revolution than before....the shaft that rotates to give the speedometer a reading is ALSO going slower...as it doesn't HAVE TO rotate as fast with a larger tire, to go the same speed.
__________________________
So the measurement devices are not "Affected" by new gears or larger tires...they just read off what ever input they were supposed to...but, are off by the percentage you changed the inputs by.
If you got larger tires....MORE distance is being covered than they thought.
If you get lower gears, LESS distance is being covered than they thought.
The REASON we GET lower gears when we get larger tires, is to try to achieve the rpm range that works for either off roading or gas mileage, or whatever our objective is.
All the posted charts are WAAAAAY out of date...as they assume a 1:1 final drive ratio (Which also makes it a simpler chart...)...whereas, there ARE NO rigs that actually HAVE a 1:1 final drive ratio...ALL rigs now a days have an overdrive top gear ratio that is what they are designed to cruise at hwy speeds with....to get better mpg.
A better way to pick gears than the charts, is to simply use the percentage your tire is larger, and see the proportional rpm drop from using the larger tire...
...and then use the ~ SAME percentage change, in deeper gearing, to RAISE the RPM by the same amount you dropped it with the tires.
IE: If the tires DROPPED THE RPM by 10%, get new gears that RAISE THE RPM by 10%....that wy, you are cruising at the same rpm at a given speed, etc.
IF you want MORE power, and LESS MPG...go deeper, and have HIGHER RPM, etc.
___________________
Hope that helps.
Essentially, is the change affecting the SPEEDOMETER, or affecting the SPEED?
For example...it seems that the JK uses a Hall Effect speed sensor that reads off of the drive shaft revolutions, to calculate the ground speed, and tone ring sensors on the wheels to calculate wheel spin/ABS/Traction Control, etc...issues.
That means that the SPEEDOMETER is calibrated to interpret a given number of SHAFT RPM as corresponding to a particular SPEED...assuming it knows what gear you are in, your tire size, and your diff ratio.
If you change the diff ratio...say with lower gearing....you raise your ENGINE RPM (Tach reading) for a given real speed...as the engine has to spin faster to make the Jeep go the same speed.
So - the new diff ratio changed the speed the engine ran at, to compensate for the shorter distance each engine revolution sent you...BUT, changing the diff ratio didn't affect the Tach itself....the Tach was just reporting the facts regarding the new corresponding ENGINE RPM.
Same with the speedometer....if the shaft it reads the speed off of, is actually going faster....it thinks the truck is going faster....if the shaft is going faster because the new gears make it HAVE to go faster, to go the same speed...the new gears didn't affect the SPEEDOMETER, they affected the READING of the SPEEDOMETER.
Does this make sense?
If it does make sense, do the same for a larger tire...as the larger tires will make that shaft spin SLOWER, as every tire revolution covers MORE DISTANCE than before.
AND - as you go further for every tire revolution....the engine doesn't have to spin as fast as it used to to go the same speed.
So the tire made the tach SHOW that the engine RPM is lower than before...because it IS slower than before.
AND
Because the tire covers more distance with every tire revolution than before....the shaft that rotates to give the speedometer a reading is ALSO going slower...as it doesn't HAVE TO rotate as fast with a larger tire, to go the same speed.
__________________________
So the measurement devices are not "Affected" by new gears or larger tires...they just read off what ever input they were supposed to...but, are off by the percentage you changed the inputs by.
If you got larger tires....MORE distance is being covered than they thought.
If you get lower gears, LESS distance is being covered than they thought.
The REASON we GET lower gears when we get larger tires, is to try to achieve the rpm range that works for either off roading or gas mileage, or whatever our objective is.
All the posted charts are WAAAAAY out of date...as they assume a 1:1 final drive ratio (Which also makes it a simpler chart...)...whereas, there ARE NO rigs that actually HAVE a 1:1 final drive ratio...ALL rigs now a days have an overdrive top gear ratio that is what they are designed to cruise at hwy speeds with....to get better mpg.
A better way to pick gears than the charts, is to simply use the percentage your tire is larger, and see the proportional rpm drop from using the larger tire...
...and then use the ~ SAME percentage change, in deeper gearing, to RAISE the RPM by the same amount you dropped it with the tires.
IE: If the tires DROPPED THE RPM by 10%, get new gears that RAISE THE RPM by 10%....that wy, you are cruising at the same rpm at a given speed, etc.
IF you want MORE power, and LESS MPG...go deeper, and have HIGHER RPM, etc.
___________________
Hope that helps.
So, if I may ask, just for clarity's sake . . .
With an automatic and currently running the 3.73s from the factory, if I were to move up to 35" tires as I intend, I would see better gas mileage on the highway with 4.56s, but less power offroad? Basically the bigger numbered gears give more leverage, but drop mileage to do it? So if I move up to 4.88s, the grunt will increase while mileage drops?
Sorry to beat a dead horse, but math and I do not get along well at all. Thanks!
JK Super Freak
Joined: May 2008
Posts: 1,279
Likes: 0
From: Grand Junction, CO
As part of this discussion; what is the optimal RPM to be turning when going down the freeway? As to what optimal is I'm thinking in terms of not having the auto trans hunt in and out of overdrive and return decent fuel economy. I've heard various things such as an RPM near of over 3000 kills your fuel economy.
I just had a set of 35x12.50x17 KM2's delivered and the lift kit is supposed to be delivered Monday. I drove a similarly equiped 4dr JK with 4.10's and it wasn't as bad as I thought it would be. But I'm thinking of going to either 5.13's or 4.88's.
I just had a set of 35x12.50x17 KM2's delivered and the lift kit is supposed to be delivered Monday. I drove a similarly equiped 4dr JK with 4.10's and it wasn't as bad as I thought it would be. But I'm thinking of going to either 5.13's or 4.88's.
JK Super Freak
Joined: May 2008
Posts: 1,279
Likes: 0
From: Grand Junction, CO
[QUOTE=RuBob;506662]here is a link to a calculator (use .69 transmission ratio for highway speeds):
http://www.4lo.com/calc/gearratio.htm
That is a pretty slick calculator at the link you posted above.
I came up withthe following:
Stock auto Rubi w 32's and 4.10's, in OD at 75 = 2228 rpm
35's and 4.10's in OD at 75 = 2037
35's and 4.56's in OD at 75 = 2265
35's and 4.88's in OD at 75 = 2424
35's and 5.13's in OD at 75 = 2545
So 4.56 would equal stock but it is a bit gutless stock and the cost would be too much for the benefit.
The 5.13's seem to be the obvious choice for an auto as you would get the biggest bang for the expenditure; unless the 2545 but you at a point where your gas milage took a big hit. But would the 120 rpm's lower if going with the 4.88's improve mpg much?
At least with the 5.13's you've prserved the ability to run 37's. The rpm at 75 with 37's and 5.13's is 2411.
I've got to decide real quick as the lift is being delivered tomorrow and I already have the wheels and tires. Being in Colorado makes the 5.13's more attractive than if I were in the flatlands.
http://www.4lo.com/calc/gearratio.htm
That is a pretty slick calculator at the link you posted above.
I came up withthe following:
Stock auto Rubi w 32's and 4.10's, in OD at 75 = 2228 rpm
35's and 4.10's in OD at 75 = 2037
35's and 4.56's in OD at 75 = 2265
35's and 4.88's in OD at 75 = 2424
35's and 5.13's in OD at 75 = 2545
So 4.56 would equal stock but it is a bit gutless stock and the cost would be too much for the benefit.
The 5.13's seem to be the obvious choice for an auto as you would get the biggest bang for the expenditure; unless the 2545 but you at a point where your gas milage took a big hit. But would the 120 rpm's lower if going with the 4.88's improve mpg much?
At least with the 5.13's you've prserved the ability to run 37's. The rpm at 75 with 37's and 5.13's is 2411.
I've got to decide real quick as the lift is being delivered tomorrow and I already have the wheels and tires. Being in Colorado makes the 5.13's more attractive than if I were in the flatlands.
JK Super Freak
Joined: Sep 2007
Posts: 1,623
Likes: 5
From: Lawrenceville, NJ
That DOES help! I, like the original poster, have little experience with swapping gears.
So, if I may ask, just for clarity's sake . . .
With an automatic and currently running the 3.73s from the factory, if I were to move up to 35" tires as I intend, I would see better gas mileage on the highway with 4.56s, but less power offroad? Basically the bigger numbered gears give more leverage, but drop mileage to do it? So if I move up to 4.88s, the grunt will increase while mileage drops?
Sorry to beat a dead horse, but math and I do not get along well at all. Thanks!
So, if I may ask, just for clarity's sake . . .
With an automatic and currently running the 3.73s from the factory, if I were to move up to 35" tires as I intend, I would see better gas mileage on the highway with 4.56s, but less power offroad? Basically the bigger numbered gears give more leverage, but drop mileage to do it? So if I move up to 4.88s, the grunt will increase while mileage drops?
Sorry to beat a dead horse, but math and I do not get along well at all. Thanks!
------------------------------------------------------------
As part of this discussion; what is the optimal RPM to be turning when going down the freeway? As to what optimal is I'm thinking in terms of not having the auto trans hunt in and out of overdrive and return decent fuel economy. I've heard various things such as an RPM near of over 3000 kills your fuel economy.
I just had a set of 35x12.50x17 KM2's delivered and the lift kit is supposed to be delivered Monday. I drove a similarly equiped 4dr JK with 4.10's and it wasn't as bad as I thought it would be. But I'm thinking of going to either 5.13's or 4.88's.
I just had a set of 35x12.50x17 KM2's delivered and the lift kit is supposed to be delivered Monday. I drove a similarly equiped 4dr JK with 4.10's and it wasn't as bad as I thought it would be. But I'm thinking of going to either 5.13's or 4.88's.
_____________________________________________
OK....
Write down these numbers...
What is your current RPM at your normal cruising speed you drive at?
IE: 2,000 rpm at 60 mph....or what ever.
Now, take the difference in the tire size you have, and the one you plan on...
IE: Have 32's, will have 35's, or whatever.
Divide the current tire size by the new size...
IE: 32/35 = 0.914 (That's 91.4%)
Take the 91.4%, and figure the difference is 8.6% between the two tire sizes. (Or whatever YOUR DIFFERENCE IS...)
--------------------------------
OK, you know that the LARGER TIRE makes fewer revolutions per mile...as its bigger around.
And, if the larger tire is bigger around, its like a bigger gear, and for the SAME NUMBER OF REVOLUTIONS, it covers more ground....so, you go faster than you did on the smaller tires.
YOU ALSO know, that the ENGINE has to make fewer revolutions to go the same distance if the tire is larger...so, the ENGINE RPM DROPS BY THE SME PERCENTAGE THAT THE TIRE REVOLUTIONS DROP....
...so if the tire revolutions drop by 8.6% because the larger tire needs fewer times around to go the same distance...the ENGINE has to turn over 8.6% fewer times to PUSH those larger tires.
If you look at the engine's power curve...it's efficiency is in the ~ 1,500 - 2,300 rpm range for economical cruising....but maximum torque is reached at closer to 4,000 rpm.
Lets say you normally drove 60 mph at 2,000 rpm...and you swapped out the 32's for 35's...your RPM would drop to ~ 1,828 RPM from 2,000 RPM. (A ~ 8.6% drop...)
If you live in Colorado's Mountains, or hilly terrain...that might be too low an rpm to maintain speed up a hill, etc....and you'd down shift...which would then raise the rpm closer to the peak torque curve areas.
If you have an automatic...it would down shift for you, etc.
When the terrain flattened out again, etc...the rpms could be dropped back to the cruising range that gives the mpg you want.
Plan B - if you swapped out the 3.73 diff gears for 4.88's...
....You'd do the SAME math as for the tires, and use 3.73/4.88 = 0.764, or 76.4%...a 23.6 % difference.
As you know a higher numerical gear makes MORE REVOLUTIONS TO COVER THE SAME DISTANCE...the OPPOSITE of a larger tire....
....the 4.88 diff gears would RAISE YOUR RPM BY THAT 23.6% difference.
-----------------------------------------
So - you started with 32's aand 3.73 diffs, and ended up with 35 and 4.88 diffs...
The TIRES DROPPED the RPM by 8.6 %, to ~ 1,828 RPM...and the 4.88's RAISED the rpm by 23.6%, or form 1,828 RPM up to ~ 2,259 RPM.
Substute YOUR NUMBERS)
Driving at 2,259 RPM instead of at about 1,828 rpm is equivalent to driving about shifted ~ gear lower, and suffering the accompanying mpg losses.
On the plus side, you are driving around closer to the power curve area, and the jeep will feel more responsive, and have better acceleration/ability to maintain speed on steep hills, etc....so you can get to the gas station faster for more of that ~ $4/gallon goodness.
Thread Starter
JK Enthusiast
Joined: Dec 2007
Posts: 134
Likes: 0
From: Mississippi
Alright I just got off the phone with David at northridge and ordered my 4.88s.
I figured the small difference from the mythical "4.56" to 4.88 wouldn't be as bad as running around with the overdrive off. Hell, I've got a heavy foot anyways so maybe the extra grunt will be nice. David said he'd ship today so hopefully they'll show up before I head back offshore so I can get them installed. I'll post my actual gains once they're installed and I reprogram everything with the hypertech. 
I figured the small difference from the mythical "4.56" to 4.88 wouldn't be as bad as running around with the overdrive off. Hell, I've got a heavy foot anyways so maybe the extra grunt will be nice. David said he'd ship today so hopefully they'll show up before I head back offshore so I can get them installed. I'll post my actual gains once they're installed and I reprogram everything with the hypertech.
Thread Starter
JK Enthusiast
Joined: Dec 2007
Posts: 134
Likes: 0
From: Mississippi
if you read my posts, you'd see that the poster with the "higher than expected" rpms had a 6-speed and your rpms would be lower because of the auto. i had already explained this then you just ignore that and say your rpm's are going to be well over 2500.
here is a link to a calculator (use .69 transmission ratio for highway speeds):
http://www.4lo.com/calc/gearratio.htm
i'm here to learn and to try to help when i can. let me know when you find a post of me gloating. thanks
here is a link to a calculator (use .69 transmission ratio for highway speeds):
http://www.4lo.com/calc/gearratio.htm
i'm here to learn and to try to help when i can. let me know when you find a post of me gloating. thanks