Gear Ratios

DirtNapLJ7

DirtNapLJ7

I DON'T know what I'm doing
Joined
Nov 13, 2020
Location
Raleigh
Can someone help this rookie out?

I’m building a DD/capable weekend warrior basically from scratch. Can anyone point me in the direction of some good reading on how decide on the right rear end ratio?

Is it as simple as using the calculator to figure what my rpm will be at highway speeds and make sure it’s not crazy high. I’m unsure of the low end considerations I need to think about?

Not gonna be rock crawling this thing. Light to medium trails and very streetable is my goal. (Disclaimer: looking for some knowledge on how to fish, not looking to be given fish:D)
 
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I've always used this one.

Gear Ratio Calculator

www.grimmjeeper.com

DirtNapLJ7 said:
Not gonna be rock crawling this thing. Light to medium trails and very streetable is my goal.
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That's how I've approached my Jeep as well and I landed on 3.73s w/ 31" tires with an AX15 trans. I do daily drive mine and feel like I could have gotten away with 3.53s for a little more highway gear.
 
Tire size, engine size, transmission, weight or body style would all be helpful.
But somewhere between 3.08 and 5.13 would work.😅

Really need to know what tire size and engine you intend to have at a minimum.

www.yukongear.com

FAQ

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www.yukongear.com

Tire Height Calculator | RANDYS Worldwide

By entering tire width, the aspect ratio, and wheel diameter this calculator spits out the height of your tire.
www.yukongear.com www.yukongear.com
 
Thanks for the insight. Was looking for more high-level information on how to make the determination on ideal engine RPM for the lower end driving which is why I left out those details at first. Anyway, here it goes:

The “Plan”:
- 5.3L LS
- most likely: 4L80e (2.48 low gear, .75 OD)
- Dreaming of: 6L80e via TCM-2650 (4.03 low gear, 0.67 OD)
- Dana 300
- 35” tires

Application is first and foremost a Daily driving pavement princess, it will see highway speeds and I don’t want the engine to be screaming or fuel mileage to suck terribly. I also do a good amount of driving on rough cut roads on the farm and trails and want to be able to handle intermediate trails. No (intentional) rock crawling.

With these numbers using the calculator I’m sitting at ~1900 rpm with 3.5 gears at 75mph. Is this all I need to worry about? Will I be happy getting 35” tires going from a stoplight with 3.5’s in the rear?

I’m just new to all this. Any advice is appreciated.
 
DirtNapLJ7 said:
Thanks for the insight. Was looking for more high-level information on how to make the determination on ideal engine RPM for the lower end driving which is why I left out those details at first. Anyway, here it goes:

The “Plan”:
- 5.3L LS
- most likely: 4L80e (2.48 low gear, .75 OD)
- Dreaming of: 6L80e via TCM-2650 (4.03 low gear, 0.67 OD)
- Dana 300
- 35” tires

Application is first and foremost a Daily driving pavement princess, it will see highway speeds and I don’t want the engine to be screaming or fuel mileage to suck terribly. I also do a good amount of driving on rough cut roads on the farm and trails and want to be able to handle intermediate trails. No (intentional) rock crawling.

With these numbers using the calculator I’m sitting at ~1900 rpm with 3.5 gears at 75mph. Is this all I need to worry about? Will I be happy getting 35” tires going from a stoplight with 3.5’s in the rear?

I’m just new to all this. Any advice is appreciated.
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There are probably a bunch of folks smarter on this than me. But I'd recommend a taller gear than 3.56 for that setup (either transmission mentioned). I've run a CJ with a fuel injected 4.2, T-18 transmission (4:1 low, 4 speed), and a Dana 300 with 4.88 gears. If the transmission had the extra two speeds like the newer ones you mentioned it would have had better highway manners. I think you will find 3.56 gearing to be a bit of a dog and you will be shifting gears on hills. You may consider 4.10 gears or 3.73.

One nice thing is that you may be able to find lots of these setups for resale due to them being standard for Jeeps and people swapping them out for higher ratios for better off-road performance.
 
4.56 or 4.88s. I run a 5.3/4l60e with 4.88s and 35s and it is perfect. 4l80 has a higher first gear, so I'd lean more towards 4.88s.
 
NCJeeplover said:
People put to much weigh into rpms at cruising. You need a little more than stock to make up for heavier everything. Tons of people told me 5.38's would suck on the highway, but my dodge cruises with the best of them. Just ask the JK guys, its not uncommon for them to have 5.13's with 35's.

Don't be scared of a little gear.
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This, I'm much more a fan of higher cruising rpms than to be lugging along and it hunt in and out of OD for small hills.

To the OP if it were me I'd go with 4.88s, puts cruising rpms at ~2500 at 70 mph. Which is in line with stock 2500HD gas trucks. I've never proven this but I'm a firm believer that higher rpm and less load is more efficient than lower rpm (lugging) and high load.
 
NCJeeplover said:
People put to much weigh into rpms at cruising. You need a little more than stock to make up for heavier everything. Tons of people told me 5.38's would suck on the highway, but my dodge cruises with the best of them. Just ask the JK guys, its not uncommon for them to have 5.13's with 35's.

Don't be scared of a little gear.
Click to expand...
Chris_Keziah said:
To the OP if it were me I'd go with 4.88s, puts cruising rpms at ~2500 at 70 mph. Which is in line with stock 2500HD gas trucks. I've never proven this but I'm a firm believer that higher rpm and less load is more efficient than lower rpm (lugging) and high load.
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Thanks for the advice guys. Good point on not wanting to be lugging around in the slower/city driving.
 
NCJeeplover said:
People put to much weigh into rpms at cruising. You need a little more than stock to make up for heavier everything. Tons of people told me 5.38's would suck on the highway, but my dodge cruises with the best of them. Just ask the JK guys, its not uncommon for them to have 5.13's with 35's.

Don't be scared of a little gear.
Click to expand...
I have a JKU with 5.13s and 35s. It is perfect on the highway. It is the 3.8 with a 42RLE transmission. So, I need the tall gears to make up for the minivan transmission.
 
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