456 vs 488 Questions

[DubaiMarauder]

I don’t think it’s that high at 80 but honestly I’d have to see. Theres a spreadsheet somewhere on the forums that does easy calcs for rpm at various speeds by inputting gear ratio and tire size, but I can’t locate it at the moment.

Im fairly certain I've seen it before, and IIRC, its somewhere north of 3k RPMS at 80mph, and just under 2800 at 70ish.

Granted 3500 may have been a bit high of an estimate, but its still not a great RPM level for fuel econ. I don't have a heavy foot and rarely take mine over 72-75mph. Maybe Im too conservative, or maybe its my 15 years behind the wheel of a semi thats gotten me used to cruising at 68-72mph without a need to be first or pass everyone to get to the head of the pack, but when im driving my favorite brick down the road, i really have no need for speed. Fuel Econ matters to me, even though Im getting 27-29mpg on the highway at 70mph.

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[dragoneggs]

Yeah, I do things like this...

Well done, crazy Roy!

 

[Gorilla57]

No. I would not. He would call you out for running the wrong gear ratio. Hows the cell reception there? He might even pull out his webpage to prove it.

What rig were you running? If it's not a stretched LJR on 43s sticky, it's trash.

The Jimmy’s buggy is on 43” stickies…….but I’m running 5.83 axle ratio with a 4.3:1 t-case. Totally wrong setup behind my LS3 and TH400.

You read that right…..5.83’s in the diffs. 🫣

 

[DubaiMarauder]

The Jimmy’s buggy is on 43” stickies…….but I’m running 5.83 axle ratio with a 4.3:1 t-case. Totally wrong setup behind my LS3 and TH400.

You read that right…..5.83’s in the diffs. 🫣

I can hear the metal of the driveshaft, axles and diffs crying from here.

 

[grimmjeeper]

I don’t think it’s that high at 80 but honestly I’d have to see. Theres a spreadsheet somewhere on the forums that does easy calcs for rpm at various speeds by inputting gear ratio and tire size, but I can’t locate it at the moment.

I know a guy with a website...

 

[JamesWyatt]

Im fairly certain I've seen it before, and IIRC, its somewhere north of 3k RPMS at 80mph, and just under 2800 at 70ish...

Nope.

The RPM charts circulating online must be recycling JK numbers.

4:88/auto/315s

 

[MA Joop]

Im fairly certain I've seen it before, and IIRC, its somewhere north of 3k RPMS at 80mph, and just under 2800 at 70ish.

Granted 3500 may have been a bit high of an estimate, but its still not a great RPM level for fuel econ. I don't have a heavy foot and rarely take mine over 72-75mph. Maybe Im too conservative, or maybe its my 15 years behind the wheel of a semi thats gotten me used to cruising at 68-72mph without a need to be first or pass everyone to get to the head of the pack, but when im driving my favorite brick down the road, i really have no need for speed. Fuel Econ matters to me, even though Im getting 27-29mpg on the highway at 70mph.

Agree with everything you said, however life is all about trade offs. I don‘t spend much time on the highway, so I’m happy to sacrifice a highway mpg or two for the off-road performance. It is a jeep, after all.

 

[MA Joop]

I know a guy with a website...

I came across your website but didn’t remember which zf transmission we have (not sure if ratios differ) and didn’t feel like looking it up.

 

[Zandcwhite]

It isn't a huge difference between the 2, but I felt even 4.56s were lower than needed for 35s. That said I do tend to drive fast on the freeway. 85mph is normal cruise control speed on road trips. I preferred the 4.10s for 35s personally. I'm guessing you don't have an 8 speed in the CJ so there's really no comparison. We have nearly twice as low a 1st gear as most older autos. Of course with the double overdrive you can get away with much lower axle gears as well which is why you'll get recommendations all over the map.

 

[JamesWyatt]

It isn't a huge difference between the 2, but I felt even 4.56s were lower than needed for 35s. That said I do tend to drive fast on the freeway. 85mph is normal cruise control speed on road trips. I preferred the 4.10s for 35s personally. I'm guessing you don't have an 8 speed in the CJ so there's really no comparison. We have nearly twice as low a 1st gear as most older autos. Of course with the double overdrive you can get away with much lower axle gears as well which is why you'll get recommendations all over the map.

Location. Vehicle weight. Engine choice. Use cases. And expectations. That's why you get so many varied answers.

If anything, the JL auto trans levels out expectations from JL owner to JL owner since it works well with almost any gear/tire combination.

 

[MA Joop]

at what RPM? 3500?

315-70-17, 8 speed auto and 4.88 gearing

 

[6.2Blazer]

I actually got a kick out of Joe's post, as I think a lot of people put way too much thought into gears. Sure, there are "optimal" gear ratios but also a lot of ratios that work just fine. I've been off-roading for 30 years and have never heard somebody say "I would have made that obstacle if I had 4.56 gears instead of just 4.10 gears...". The driver plays a much, much bigger role in how a vehicle performs as compared to axle gears. Sure, you don't want 3.07 gears with 40" tires, but something like that is an extreme case.
So for the original post. There is simply not a huge difference between 4.56 and 4.88 gears. At 60 mph there is maybe 120 rpm difference. If you put most people in one Jeep with the tach covered up with 4.56 gears and drove it around, then put them in an otherwise identical Jeep with 4.88 gears....and didn't tell them they had different gears....I bet most couldn't tell any difference. 4.10 vs. 4.88 is usually a big enough jump to feel, but still not what I would call extreme.
According to a gear calculator running 80 mph in an 8 speed auto with 35" tires is around 2,600 rpm.

 

[CptFloridaMan]

It isn't a huge difference between the 2, but I felt even 4.56s were lower than needed for 35s. That said I do tend to drive fast on the freeway. 85mph is normal cruise control speed on road trips. I preferred the 4.10s for 35s personally. I'm guessing you don't have an 8 speed in the CJ so there's really no comparison. We have nearly twice as low a 1st gear as most older autos. Of course with the double overdrive you can get away with much lower axle gears as well which is why you'll get recommendations all over the map.

I tend to agree after driving 4.56s and 35s for a bit. For me trans tuning matters more now since it has a tendency to just keep the jeep in the highest gear. So that’s gonna be my next venture i think.

 

[Coreyb516]

Go with the 4.88s. My 2023 JLUR has 4.88s with the factory 33s and I love the gearing. 35s would probably be perfect with 4.88s

Do you have an automatic or a manual what motor do you have?

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