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RPM's to low on Highway

4K views 13 replies 9 participants last post by  Quest4life 
#1 ·
How can I adjust my RPM so when I am on the highway I can run around 2600 RPM. I currently run 2200 @ 72 MPH. Just nor enough power that low. If I shut the overdrive off it runs @ 3000. I think 2500 to 2600 would be perfect.
I have an 03 TJ with the 4.0 Auto trans. I have 4:10 gears and 35 inch tires.
That is about all the info I have. I'm not sure what trans it has. From the factory it had 3:73 gears. I installed front and rear axles from an 05 Rubicon and it had the 4:10 gears and I did change the gear in the tranny so the speedo is correct.
Thanks for any help.
 
#5 ·
Looking at a chart of rpm vs hp for our 4.0 engines is eye-opening but discouraging for those searching for a lot of power from regearing.

At about 2200 rpm, the 4.0 is producing about 90 hp. This sounds anemic, but at 2600 rpm it rises only to about 110 hp. Even at 3000 rpm it's only to about 130 hp.

Not an enormous difference at around 20 to 40 hp.

Shucks, turning on the air conditioner might take 10 of those horses by itself. No wonder it's popular to go topless.

So, even though the engine gains power at the higher rpms being discussed here, the difference is only a relatively small improvement. And it takes going up to an impractical-to-sustain 4600 rpm to get to the peak of about 190 hp from our engines.

For those who say it's all about the torque produced not the horsepower, note: Amongst all the rpm variations discussed here, the torque difference is really, really small, ranging from 220 lb ft at 2200 rpm to 230 lb ft at 3000 rpm, and then actually drops to 215 lb ft when the horsepower peaks at 4600 rpm.

Yep, these ole tractor engines are fine for providing a respectable grunt on the trail at a crawling pace, but they're nothing to brag about trying to push a barn door down the highway at higher speeds, especially when they're laden with gigantic tires.
 
#13 ·
At about 2200 rpm, the 4.0 is producing about 90 hp. This sounds anemic, but at 2600 rpm it rises only to about 110 hp. Even at 3000 rpm it's only to about 130 hp.
That's only looking at half the picture. If someone regears from 4.10 to 5.38 to GET the RPM range to move, they're ALSO increasing the mechanical advantage between the engine and wheels by 30%, which is a significant change. So in addition to the engine making a bit more power/torque, it is also able to exert significantly more force at the end of the drivetrain. That improves acceleration and makes it easier to maintain speed on hills.
 
#6 ·
What the numbers reflect is that the engine is more efficient at lower RPM. Keeping mind the more RPMs your motor turns over a given mileage, the shorter your engine life is. As Airstreamer points out, Jeeps aren't monster trucks. If you want to go that direction either turbo or supercharge or drop a V8 in along with all the other pertinent goodies. I don't mean that facetiously either. You can turn these into massive trucks, but that's not what they were made to be per se. I spend a lot of time in the woods and the ability of the Jeep is its small size, light weight and tight turning radius. Naturally there is a whole level of play that turns these things into something else and then there is also aesthetics.

Both are fine, but to answer your question your option is to regear (there are plenty of calculators to determine speed at a given RPM, final and tire) or run shorter tires.
 
#7 ·
The 42RLE is a special beast. Only those of us unfortunate enough to have this transmission in our rigs with 35"+ tires can truly understand the challenges we face at highway speeds. Next time around I plan to go with 5.89 gears instead of 5.38.

The one positive aspects of the 42RLE is the ability to install a Rubicrawler. My rig has 114:1 ratio in 4low.
 
#10 ·
Quest4life, in what way did your Jeep perform better with the 3.73s compared with the 4.10s?

The lower 4.10 gear ratio will increase the RPMs about 10% or so, which will raise the available horsepower at any given road speed. Although this is not a large amount as noted earlier, the performance certainly should not feel like it is worse.

So why would you feel the performance dropped with the gear change?
 
#11 ·
Because that auto has such a wide step (% change ) between 3rd and OD

The small 10 or 11% change in axle ratio does not make that rpm Increase at all speeds as the shift to OD point also moves to a lower speed due to the numerically higher gears


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#14 ·
Quest4life, in what way did your Jeep perform better with the 3.73s compared with the 4.10s?

The lower 4.10 gear ratio will increase the RPMs about 10% or so, which will raise the available horsepower at any given road speed. Although this is not a large amount as noted earlier, the performance certainly should not feel like it is worse.

So why would you feel the performance dropped with the gear change?

It seemed to shift better, it didn't drop so low , it just seemed smoother. I think it would be fine if it dropped down to 2500 to 2600 rather than 2,200
 
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