4.88 or 5.13?

[DesertsJL]

This is going to sound dumb, but I like having a tall gear ratio. In my JK I ran 32” tires and I had it geared to 4.88. Mostly because it ran like a dog, but also so that the engine wasn’t working so hard. My mileage was actually helped by the gear change because I’m rarely on the freeway. Around town the taller gears work better by getting up to speed and getting the heavier rolling weight moving. This new JL has so much overdrive that I’m thinking of running tall gears again even though I’m only going to 35’s. I did the calculator and at 70 I’d only be running 2310rpm at 70mph if I went to 5.13 gears, plus it’d give me plenty of headroom if I wanted to upgrade to a taller tire later. At 4.88’s I’d be running 2197rpm.

Am I stupid, or does anybody else see the value in running such a tall gear for a relatively small upgrade in tire size on their new JL?

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[Sting_NC_USA]

Not stupid at all. I see you've used a calculator, but have you seen this type of chart? These serve as great guides because you can see the bands that correspond with various driving styles. This one is from Quadtratec and supports your specific goal, especially since you aren't worried about highway driving. Pic below and link here.

 

[DesertsJL]

I’ve seen those charts. The problem in them is that they usually show a 1:1 number and don’t account for the overdrive gears. They also show reduced economy and I think that they are assuming freeway driving. In stop and go or around town driving, higher gears increase economy, at least they have for me. Less throttle, less fuel.

 

[Jeepsk8]

I'm not saying you're wrong, there are lots of variables to take into account. If nothing else changes on the vehicle, then the premise that turning more rpms for a given speed are going to reduce your fuel consumption does't really compute. Yes. taller gears will get you up to speed faster, but for any given speed (including acceleration) you are turning more rpm's, more rpm's require more fuel to turn those rpm's. The torque multiplication of the taller gears reduces load on the engine, I get that, but a completely unloaded engine still requires more fuel to increase speed.

 

[intentsrig]

I’m doing 4.88’s with 35’s. I’d go deeper but have the dana 30...

 

[GARRIGA]

Isn’t mpg mostly about rpms and wind resistance? My Hemi Durango gets 30 mpg at 45 in a lower gear but still turning around 2000 something rpms yet at 80 and similar rpms on higher gear (OD) gets 20. I’ve concluded it’s the added wind resistance. Can’t think of anything else. If I had taller gears at 45 I’d be in a higher gear but at 80 I’d lose the most because I’m already on OD and just higher rpms and same wind resistance.

 

[DesertsJL]

My mileage is mostly from running around town, not the freeway. If I regear to a taller gear the motor isn’t working as hard to get the JL or the rolling mass of the bigger wheels and tires moving. It’s the stop and go that matters, plus the fact that it will run in a higher gear should be able to make full use of all 8 gears.

 

[Jeepsk8]

From what I have been reading the sweet spot for 37's is a 4.56 gear on the JL with an auto. Any taller gear will definitely get you a better crawl ratio if that's important to you, but you suffer in the top end. As far as stop and go traffic, that is going to suck for mpg regardless of what gear ratio you have. And yes, wind resistance is the largest factor in cruising speed mpg for any vehicle. Let's face it, a Jeep is a rolling brick with a tent tied to it, to hope for good fuel economy is kind of foolish.

For me, I'm sticking with the stock 4.10 gears. It does OK on gas (for a Jeep) with the stock tires. I bolt on 35's and tow my lightweight camper when it's time to go wheeling. Towing sucks the mpg down too, so either way it's a gas hog when it's time to have fun.

 

[RubiRob]

OP: What did you end up getting?

 

[butcho]

OP: What did you end up getting?

He went 5.38

Let’s say that I only wanted to keep the original relative gear ratio between the stock tire and gear and the larger new tire and gear. That would mean that roughly a 4.35 gear would be needed for 35’s and 4.60 gears would be needed for 37” tires. The problem is that with larger tires you also have much more rotating mass, necessitating a deeper gear than the calculator would reflect. You need more gear in order to control and move the extra weight of the tires.

My suggestion is to go at least one step deeper than the calculator would suggest. For 35” tires use 4.88 and for 37” tires use 5.13 gears.

For me, I went very deep with 5.38 gears and currently 35” tires, admittedly a little too deep. However I live in a mountainous area and rarely drive over 70mph. Due to the super tall 8th gear of a .67 overdrive, at 70mph I’m running 2450rpm. My plans are to run a larger tire down the road.

 

[Litfuse]

Just for the record, going to any higher numerical number is actually considered lower gears because...oh heck, I’m not going to explain it.

 

[vavaroutsos]

Just for the record, going to any higher numerical number is actually considered lower gears because...oh heck, I’m not going to explain it.

Yes, higher number gear ratio is a shorter gear. Lower number gear ratio is a taller gear.

 

[Jeepsterfreak]

I’m doing 4.88’s with 35’s. I’d go deeper but have the dana 30...

DANA Spicer lists up to 5.13 for the JL Dana 30/35.

 

[mickey3]

This is going to sound dumb, but I like having a tall gear ratio. In my JK I ran 32” tires and I had it geared to 4.88. Mostly because it ran like a dog, but also so that the engine wasn’t working so hard. My mileage was actually helped by the gear change because I’m rarely on the freeway. Around town the taller gears work better by getting up to speed and getting the heavier rolling weight moving. This new JL has so much overdrive that I’m thinking of running tall gears again even though I’m only going to 35’s. I did the calculator and at 70 I’d only be running 2310rpm at 70mph if I went to 5.13 gears, plus it’d give me plenty of headroom if I wanted to upgrade to a taller tire later. At 4.88’s I’d be running 2197rpm.

Am I stupid, or does anybody else see the value in running such a tall gear for a relatively small upgrade in tire size on their new JL?

I do not think you are stupid, I agree with everything you said. I too prefer deep gears in my vehicles. I am currently running 5.13's In a JLUR 6 speed with 35's. With two overdrive gears in Jeeps JLs they are still easy to drive on the interstate even with 5.13's. Why not consider 5.38's? cause you know your going to run 37's someday, maybe even 40's, plus gears make for a great foundation for other mods.

 

[vavaroutsos]

You should use the dynamic tire diameter for RPM calculations, not static. Your RPM will be higher than you calculated. The higher gear ratios have weaker pinions, so I wouldn't go 5.38 unless you're sure you need it later.

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