Rubicon Owners Forum banner

1 - 18 of 18 Posts

·
Registered
Joined
·
303 Posts
Discussion Starter #1
I upgraded to 35x12.5x15 BFG's. I still have the stock 4.10 gears on my 2004 Rubi. It rides fine and doesnt feel too different than with the 33's I had before.

Question is, are my RPM's in a good place at 65mpg and 70mpg, see images.

Thanks...
 

Attachments

·
Moderator
Joined
·
904 Posts
65 and 70 miles per gallon .... holy crap!!! haha :p. Do you have an AT or MT transmission? Those seem low if you're in 5/6th gear or over drive. I wouldn't want to be in the Mountains with those rpms. I have "33's" with the stock 4.10 gears and 5speed and it's tolerable but once I get into the mountain roads of Colorado it becomes a real dog and I often found myself in 3rd gear just trying to keep up with highway traffic. I've thought about regearing to 4.56 but am holding off until next year cause I might just pull the trigger and go to 35's and be done with it.

I'd personally look into regearing. I usually shift around 2.5k rpm's for smoother shifts and the engine seems happier when it's at or just above 2k cruising around. It may not be the most fuel efficient way to drive it but when my 4cyl honda feels like it has more umph than my heep it's a sad feeling haha.
 

·
Registered
Joined
·
303 Posts
Discussion Starter #4
I have an AT. Those rpm's are on flat road. On hills rpms spike as it downshifts as expected. I am thinking of jumping to 4.56 gears or 4.88. But 4.56 seems like it would be a sweet spot and 4.88 may be a hair too much. I guess it depends on the weight of the Jeep too is what i hear.
 

·
Registered
Joined
·
1,706 Posts
Go with at least with 4.88, but for an auto 5.13 would be ideal, that what I (and most of the crowd running 35s on an auto) have.
 

·
Premium Member
Joined
·
1,335 Posts
It's personal preference to a large degree. With the auto I'd do 5.13 or 5.37, myself. Have never regretted 4.88 with 35s and six speed. Which is closer to 5.37 with the auto. And I routinely run 80mph on the interstate.

- DAA
 

·
Registered
Joined
·
395 Posts
I upgraded to 35x12.5x15 BFG's. I still have the stock 4.10 gears on my 2004 Rubi. It rides fine and doesnt feel too different than with the 33's I had before.

Question is, are my RPM's in a good place at 65mpg and 70mpg, see images.

Thanks...
Have the speedometer readings been corrected for the tire change(s) ?
 

·
Registered
Joined
·
175 Posts
Back to the original question, are the RPM's good? No, that's terrible. Just keep the OD turned off for now. That's what I did for the first year or so when I went from 33 to 35's. I eventually went to 5.13 to be able to use OD again, much better now. I believe 2500 rpm is 73 mph.
 

·
Premium Member
Joined
·
4,042 Posts
I have an AT. Those rpm's are on flat road. On hills rpms spike as it downshifts as expected. I am thinking of jumping to 4.56 gears or 4.88. But 4.56 seems like it would be a sweet spot and 4.88 may be a hair too much. I guess it depends on the weight of the Jeep too is what i hear.
As stated by others... with an Auto tranny and 35's, get 5.13 gears.
 

·
Registered
Joined
·
771 Posts
I upgraded to 35x12.5x15 BFG's. I still have the stock 4.10 gears on my 2004 Rubi. It rides fine and doesnt feel too different than with the 33's I had before.

Question is, are my RPM's in a good place at 65mpg and 70mpg, see images.

Thanks...
Too low, The oil system on the 4.0 is ancient and was designed when engines were spinning at 3000 regularly with 28 inch tires and 4.27 geas. Spinning that engine a bit faster is better and if you don't regularly get it to 4000 it can become a problem with stuck lifters, valves and pushrods wearing rockers or stuck valves . The valves don't start to rotate until 3000 rpm, and can cause some stuck or burned valves if they arnt rotating.

peak torque on the 4.0 is around 3200-3400, 75- 80% of that should be cruise RPM 2500-2700 would be my target.

4.88 at a minimum 5.13s better.
 

·
Sarcastic Moderator
Joined
·
13,994 Posts
really depends on the transmission
5 speed will be a dog, was forced to regear the 04.
6 speed is ok, but you will have to downshift to 5th on some hills.
Auto say goodbye to overdrive without a strong tailwind.
 

·
Registered
Joined
·
303 Posts
Discussion Starter #13
Thanks for all the responses. To answer some questions, yes the speedo was corrected with the speedo healer. I do plan to regear but havent decided to what ratio yet. I plan to add a few more skids. Oil pan for sure and debating on replacing the shovel.

I was thinking of doing the gears after the skids so I can make a better determination with the weight of the Jeep.

I ahould consider that right?

I am going on a long road trip in 2021, the Baja XL, 4,000 km from Los Angeles to Cabo San Lucas, then back to LA. Trying to prep the Jeep for that. Its an organized event but i dont want to have to rely on them if there is trouble so i wanted to build it up to reduce the likelyhood of trouble.
 

·
Registered
Joined
·
771 Posts
Thanks for all the responses. T...…………. Oil pan for sure and debating on replacing the shovel.

I was thinking of doing the gears after the skids so I can make a better determination with the weight of the Jeep.

I ahould consider that right?
,,,,,,,,,,,,,,,,,,,,,,,,
.
On the shovel there is no debating it should be replaced , factory recognized it and tucked the trans up in the TJs successor JK. Its a must do in my view.

Weight doesn't matter at all . Rpms and engine torque band vs final drive is all that matters. You want to have the 4,0 in its torque band while cruising to allow it to push those extra pounds, the only way to compensate for more weight is to utilize the engine at its most efficient power/ torque range, gearing is the way to correct the engine speed(rpm) for the tire size and get the engine producing hp/tq to the ground to push the weight.

Well I should say without adding power to the engine the only way to compensate for weight is to make sure the engine is running in its power band
 
  • Like
Reactions: johncanfield

·
Registered
Joined
·
38 Posts
Typically a stock TJ Rubi will be lugging in OD with bigger than 33 tires. New gears or smaller tires would be a good plan. This is at sea level with mostly on the flat. If you are talking some elevation or a lot of mountains/hilly country, then yes the new gears would go double. Try running with the OD switched Off. Should be better for the engine, trans and your mileage. 35s over the stock OEM tires is already like an overdrive. Later Glenn 04 Rubi
 

·
Registered
Joined
·
24 Posts
Currie Enterprises installed 4.88 gears in our 2003 Rubicon with a Kenne Bell supercharged 4.0 liter motor, automatic transmission, and 35" BFG 315/75R16 All Terrain tires. IMHO 4.56 would have been better since the KB supercharger adds a LOT of torque AND horsepower making the annoying out-of-overdrive downshifts on every overpass less likely. Also, the 4.88 gears kick in the Kenne Bell supercharger boost fuel map ~ 70 mph which makes the MPGs go down like Stormy Daniels in a strip club. Well OK not THAT bad but still drops 3+ MPGs vs. 65 mph which OMHO makes fuel range too short with the stock fuel tank.

I think 5.13 or 4.88 would be better for non-boosted 4.0 liter motor + AT + 35" tires... but I don't have a "stock" 4.0 liter so just extrapolating.
 
B

·
Guest
Joined
·
0 Posts
Stock tune is horrible for highway cruising. I could not use OD and it felt like 50% throttle made the same power as 75% throttle. It seemed to lose power past 75% throttle. For some reason these engines did not have knock sensors. To ensure that the engine did not ping, they pulled tons of timing when loads goes up. With stock gears and 35s, even small hills would slow me below 70 mph. OD was not an option.

After altering the timing tables, I use OD on the interstate except in hilly areas. I can now accelerate up hills on the interstate that would slow me down. Tuning will not in anyway replace a gear change, but will help no matter what gears or tires you have. I have 5.38s to go in it. They should help a ton. 70 mph will be just under 2500 rpm, which like others have said, that is where the 4.0 wakes up.
 
1 - 18 of 18 Posts
Top