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Im not sure if the rpm readings would be equivalent for a 2.5L compared to a 4.0L but thank.Originally Posted by Jim Malcolm
I'd be more than happy to do those speed tests for ya, but my XJ is lacking a steering wheel, brake and gas pedal at the moment, so, ... I also haven't yet corrected the speedometer, so it would only be a guess. The calculators should get it darn close, but remember that a 31" tire very rarely ever is 31". Some 31s I was looking at just the other day only measure 30.7", when brand new. Mine have a couple years worth of wear on them.
Very true about 31" tires not measuring 31 inches. I had the wrong speedometer for years in my XJ. I bought the gear that "matched" 31s and 3.55 gears; the tooth count I can't recall. The speedometer was off by about 5mph and it didn't shift into OD at 45mph how it use to; more like 48mph. Finally after about 10 years later I installed a 30" tire to 3.55 speedometer gear and that one is reading accurate. I even verified with a speedometer app. I don't know if that one inch less of difference is also true for larger diameter tires but it's something consider when trying to find the right gear tooth count.
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3.55s vs 4.10s is 4.10 / 3.55 = 1.15, 15%. Now we're getting somewhere... The equivalent of 30 hp and 35 ft-lb of torque from the motor.
3.55 vs 4.56 is 4.56 / 3.55 = 1.28, 28% Equivalent to 55 hp and 65 ft-lb.
I didn't calculate the numbers, but order-of-magnitude here, we're talking about an 100-200 rpm increase going from 3.55s to 3.73s and 200-400rpm going from 3.55s to 4.10s. In city driving, those numbers are meaningless because you're not driving at speed for any length of time, you're accelerating between speeds more often than not. When accelerating, your driving the throttle, not the speedometer. It's in this realm that power is king. Swapping in a larger motor generally results in less fuel usage if driven exactly the same. The exactly qualifier there is key. More motor, or more gear ratio, will take less fuel to achieve the same final force to the road, the same acceleration. That's a long way to say, if you don't have to press the pedal so hard on your 4.0, you'll save gas even with the higher RPMs.
On the highway, when you're just turning RPMs, your fuel economy will drop a bit. The math works the same, just in reverse. With 3.73s, you get 5% more power, but you also turn 5% higher rpms at any given speed. 15% for 4.10s, 28% for 4.56s. It shouldn't be too far off to use those numbers directly against fuel economy, so assuming you're getting the 20 mpg the XJ is rated for on the highway, you'd drop to 19 mpg with 3.73s, 17 mpg with 4.10s or 14.4 with 4.56s. Add in the increased rolling resistance of the bigger tires, and the increased air resistance from the vehicle sitting higher, and actual MPGs will likely be lower. The price we pay for living the dream...
IMO, 3.73s aren't worth the time and effort. The 5% power improvement is not going to make enough of a difference to solve your power issue.
It's all mind boggling but I can make some sense of it. I assume the same percentage difference in the gears would also work for tires but the calculations would be at a disadvantage; a loss of power.Originally Posted by Jim Malcolm
Since it's all multiplication/division and we're only looking at the axle gear ratio, we can use simply percentages to see what the difference in performance is going to be. The difference between 3.55s and 3.73s is going to be 3.73 / 3.55 = 1.05, 5%. All that work for 5% improvement. Roughly equivalent to adding 9 hp / 12 ft-lb to the motor.3.55s vs 4.10s is 4.10 / 3.55 = 1.15, 15%. Now we're getting somewhere... The equivalent of 30 hp and 35 ft-lb of torque from the motor.
3.55 vs 4.56 is 4.56 / 3.55 = 1.28, 28% Equivalent to 55 hp and 65 ft-lb.
I didn't calculate the numbers, but order-of-magnitude here, we're talking about an 100-200 rpm increase going from 3.55s to 3.73s and 200-400rpm going from 3.55s to 4.10s. In city driving, those numbers are meaningless because you're not driving at speed for any length of time, you're accelerating between speeds more often than not. When accelerating, your driving the throttle, not the speedometer. It's in this realm that power is king. Swapping in a larger motor generally results in less fuel usage if driven exactly the same. The exactly qualifier there is key. More motor, or more gear ratio, will take less fuel to achieve the same final force to the road, the same acceleration. That's a long way to say, if you don't have to press the pedal so hard on your 4.0, you'll save gas even with the higher RPMs.
On the highway, when you're just turning RPMs, your fuel economy will drop a bit. The math works the same, just in reverse. With 3.73s, you get 5% more power, but you also turn 5% higher rpms at any given speed. 15% for 4.10s, 28% for 4.56s. It shouldn't be too far off to use those numbers directly against fuel economy, so assuming you're getting the 20 mpg the XJ is rated for on the highway, you'd drop to 19 mpg with 3.73s, 17 mpg with 4.10s or 14.4 with 4.56s. Add in the increased rolling resistance of the bigger tires, and the increased air resistance from the vehicle sitting higher, and actual MPGs will likely be lower. The price we pay for living the dream...
IMO, 3.73s aren't worth the time and effort. The 5% power improvement is not going to make enough of a difference to solve your power issue.
For example, on stock 3.55 gears: 31" tires compared to stock 27" tires (215/75R15) that is 31/27 = 15% (rounded up) loss of power, maybe?
So putting 3.73 gears at a 5% power increase will bring the tire loss of power down to 10% ?
Or, with 4.10s being a 15% power increase it would cancel out the 15% tire loss of power and pretty much be back to normal (stock)?
Maybe Im just making this all up in my head. If that were true about 4.10s canceling out the 31" tire power loss, then would it be something in the realm of a quote like, "this 4.10 and 31" combination is almost equivalent to 3.55 and 27" stock gearing and tires. The only difference now is that with your new (4.10s,31") 'equivalent to stock' (3.55s,27") setup is the RPMs are going to be higher. This is due to the transmission gear ratios being designed for 3.55 gears with 27" tires and not 4.10 gears and 31inch tires." Something like that?
Im still fairly new to this so thanks for everyone chiming in.
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LOL yeah Ive noticed the skyrocket too. I hate when that happens and that is the reason I always downshift myself about 100ft coming up to a bridge or when necessary. Originally Posted by bluejeep2001
Yes but when it downshifts on its own it almost never locks and the rpm skyrockets...if you downshift the Tranny will relock in 3 restoring efficiency
Senior Member
You got it. Increasing tire size by a certain percentage offsets increasing the gear ratio. 5% larger tires reduce the 15% improvement from 4.10 gears, resulting in a 10% improvement in power. 15% larger tires offset a 15% gear change, resulting in EXACTLY the same RPMs at any given speed. The issue then is the increased resistance from the larger tires, the increased air resistance from sitting higher, the increased rotational weight from the larger tires, etc, etc. It's my understanding that 32s and 4.10s is 1:1 equivalent with the stock gearing and tires. Because of the "other stuff", 31s and 4.10s is a very good combo.
I didn't take notice that you have a 4-banger. Are you absolutely sure you don't already have 4.10s?
I didn't take notice that you have a 4-banger. Are you absolutely sure you don't already have 4.10s?
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I didn't take notice that you have a 4-banger. Are you absolutely sure you don't already have 4.10s?
I do have a 4.0L and 3.55s but I must have gotten confused when you said 4 banger. I wasn't sure if you were referring to your XJ with 35s or 31s. I was mostly concerned if the 2.5 would give a different rpm reading at 60 and 70 as I asked for compared to a 4.0. That's why I was hinting if it's not a 4.0 you have maybe you could skip the test but thanks for the effort. Originally Posted by Jim Malcolm
You got it. Increasing tire size by a certain percentage offsets increasing the gear ratio. 5% larger tires reduce the 15% improvement from 4.10 gears, resulting in a 10% improvement in power. 15% larger tires offset a 15% gear change, resulting in EXACTLY the same RPMs at any given speed. The issue then is the increased resistance from the larger tires, the increased air resistance from sitting higher, the increased rotational weight from the larger tires, etc, etc. It's my understanding that 32s and 4.10s is 1:1 equivalent with the stock gearing and tires. Because of the "other stuff", 31s and 4.10s is a very good combo.I didn't take notice that you have a 4-banger. Are you absolutely sure you don't already have 4.10s?
Thank you for the help in getting a better understanding about all these ratios; Very much appreciated. Im starting to see why 4.10 might be the better gears and see now why 3.73 might just not be worth it.
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4.56 gears are ok with 35s but I would rather have 5.13s. The reason I stayed with 4.56 gears is because the pinion gear on the Dana 30 front axle is very small with a 4.88 or 5.13 gear set so it can break easier, 4.56 gears are much stronger. 4.56 gears with 31s and a 4.0 would be a great setup, I would go with 4.10s at the very least, going from 3.55s to 3.73s won't make enough difference to be worth doing. Originally Posted by 2000XJmoredoor
What is pointless about 3.55 to 3.73 and what about 4.10? I see you have 4.56 gears but pushing 35s not 31s so, could you explain what would be beneficial about 4.56 gears pushing 31s?
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Thank you for the help in getting a better understanding about all these ratios; Very much appreciated. Im starting to see why 4.10 might be the better gears and see now why 3.73 might just not be worth it.
Gear ratio is what decides your RPM, a 2.5, 4.0 or any other engine is going to run at the exact same RPM if the gearing and tire size is the same. Also you don't have to test anything to see what your RPMs will be, you can do the math.Originally Posted by 2000XJmoredoor
I do have a 4.0L and 3.55s but I must have gotten confused when you said 4 banger. I wasn't sure if you were referring to your XJ with 35s or 31s. I was mostly concerned if the 2.5 would give a different rpm reading at 60 and 70 as I asked for compared to a 4.0. That's why I was hinting if it's not a 4.0 you have maybe you could skip the test but thanks for the effort.Thank you for the help in getting a better understanding about all these ratios; Very much appreciated. Im starting to see why 4.10 might be the better gears and see now why 3.73 might just not be worth it.
RPM= Speed x axle gear ratio x transmission gear ratio x 336 divided by tire diameter.
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Since it's all multiplication/division and we're only looking at the axle gear ratio, we can use simply percentages to see what the difference in performance is going to be. The difference between 3.55s and 3.73s is going to be 3.73 / 3.55 = 1.05, 5%. All that work for 5% improvement. Roughly equivalent to adding 9 hp / 12 ft-lb to the motor.
3.55s vs 4.10s is 4.10 / 3.55 = 1.15, 15%. Now we're getting somewhere... The equivalent of 30 hp and 35 ft-lb of torque from the motor.
3.55 vs 4.56 is 4.56 / 3.55 = 1.28, 28% Equivalent to 55 hp and 65 ft-lb.
Gearing doesn't change your HP at all, it just increases your final output torque. Originally Posted by Jim Malcolm
Since it's all multiplication/division and we're only looking at the axle gear ratio, we can use simply percentages to see what the difference in performance is going to be. The difference between 3.55s and 3.73s is going to be 3.73 / 3.55 = 1.05, 5%. All that work for 5% improvement. Roughly equivalent to adding 9 hp / 12 ft-lb to the motor.
3.55s vs 4.10s is 4.10 / 3.55 = 1.15, 15%. Now we're getting somewhere... The equivalent of 30 hp and 35 ft-lb of torque from the motor.
3.55 vs 4.56 is 4.56 / 3.55 = 1.28, 28% Equivalent to 55 hp and 65 ft-lb.
Senior Member
Changing gearing affects final to-the-ground force same as increasing torque produced by the engine. "Equivalent to"...
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Equivalent to more torque yes, but not more HP. Originally Posted by Jim Malcolm
Changing gearing affects final to-the-ground force same as increasing torque produced by the engine. "Equivalent to"...
Senior Member
Thanks for pointing that out. My reading comprehension has been crap today. That's when you know it's a good day off work... :-)
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RPM= Speed x axle gear ratio x transmission gear ratio x 336 divided by tire diameter.
Thanks that gives a pretty good approximation to get an idea. I'll have to use my scan tool to get a more accurate rpm reading compared to looking at the factory tach.Originally Posted by 00t444e
Gear ratio is what decides your RPM, a 2.5, 4.0 or any other engine is going to run at the exact same RPM if the gearing and tire size is the same. Also you don't have to test anything to see what your RPMs will be, you can do the math.RPM= Speed x axle gear ratio x transmission gear ratio x 336 divided by tire diameter.
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- Join DateAug 2018
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I run 4.88 with 33 and 35 tires i dont notice any difference for what its worth my jeep gets rode hard at times and ive yet break the little pinion gear.
there should only be one gear available after market for jeeps the 4.56. That way the decision becomes much easier and people quit waisting there time.
I to would like to get 5.37s
there should only be one gear available after market for jeeps the 4.56. That way the decision becomes much easier and people quit waisting there time.
I to would like to get 5.37s
Well I think I have decided on 4.10 gears instead of the 3.73. I have feeling if I went 3.73 Id soon regret not going with 4.10. Hopefully it's not vice versa.
I do have a few questions, if someone would like to give their thoughts.
Im currently running an XJ front driveshaft for the rear axle. Would I have to get a larger diameter driveshaft to compensate for the 4.10 gears? My concern is if the gears give more torque would I need a larger diameter DS to avoid any risk of twisting the XJ front shaft I am currently running.
Also, would it be easier to swap gears if the axle was off the Jeep? And for the rear does anyone know if the 4.10 ring gear teeth would have to be filed down to clear the cross pin shaft?
I do have a few questions, if someone would like to give their thoughts.
Im currently running an XJ front driveshaft for the rear axle. Would I have to get a larger diameter driveshaft to compensate for the 4.10 gears? My concern is if the gears give more torque would I need a larger diameter DS to avoid any risk of twisting the XJ front shaft I am currently running.
Also, would it be easier to swap gears if the axle was off the Jeep? And for the rear does anyone know if the 4.10 ring gear teeth would have to be filed down to clear the cross pin shaft?
Senior Member
No need for a new driveshaft since the output from the engine hasn't changed, only the torque applied to the wheels because of the increased ratio. If worked backwards, any torque applied to the tires and send back through the axle to the driveshaft is reduced by the higher gear ratio, so no, no need for a new driveshaft...
If the mechanic has a large bench bolted to the floor that'll hold the axle while the 150 ft-lbs is applied to crush the Crush Sleeve, then it's probably easier off the vehicle. If they don't, it's probably easier on the vehicle. I'd ask whomever you are having install the gears which way they prefer it.
No modifications to the Cross Pin or Gears needed for 4.10s; it clears just fine...
If the mechanic has a large bench bolted to the floor that'll hold the axle while the 150 ft-lbs is applied to crush the Crush Sleeve, then it's probably easier off the vehicle. If they don't, it's probably easier on the vehicle. I'd ask whomever you are having install the gears which way they prefer it.
No modifications to the Cross Pin or Gears needed for 4.10s; it clears just fine...
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If the mechanic has a large bench bolted to the floor that'll hold the axle while the 150 ft-lbs is applied to crush the Crush Sleeve, then it's probably easier off the vehicle. If they don't, it's probably easier on the vehicle. I'd ask whomever you are having install the gears which way they prefer it.
No modifications to the Cross Pin or Gears needed for 4.10s; it clears just fine...
Thank you for all that!Originally Posted by Jim Malcolm
No need for a new driveshaft since the output from the engine hasn't changed, only the torque applied to the wheels because of the increased ratio. If worked backwards, any torque applied to the tires and send back through the axle to the driveshaft is reduced by the higher gear ratio, so no, no need for a new driveshaft...If the mechanic has a large bench bolted to the floor that'll hold the axle while the 150 ft-lbs is applied to crush the Crush Sleeve, then it's probably easier off the vehicle. If they don't, it's probably easier on the vehicle. I'd ask whomever you are having install the gears which way they prefer it.
No modifications to the Cross Pin or Gears needed for 4.10s; it clears just fine...
The install will be sometime in the near future, after much needed body work and paint, but I'll be tackling the re-gear myself. Isn't the crush sleeve rated at 200lbs to crush? Also what about crush sleeve eliminators? A Dana30 HP has shims so maybe the Chrysler 8.25 has a kit?