Gearing Up: Choosing the proper gears for your 4×4 is an important decision
Over the last 6 months I’ve been toying with the idea of regearing my Jeep CJ7. The CJ came stock with some really high (numerically low) 2.73:1 ring and pinion gears in the axles. In ‘86 Jeep was trying to give the CJ a fuel-efficiency boost by putting these gears in the front and rear pumpkins matched with a 5 speed transmission. While my jeep does get surprisingly good mileage, it is a bit of a dog when doing freeway passing, and the gears are really too tall to do any sort of mildly difficult off-roading without riding the clutch a bit.
My Jeep now has a more modern fuel injected 4.0 motor installed in it and gets roughly 18mpg around town and 20-24mpg highway. But I find that driving on the freeway I spend as much if not more time in 4th gear than I do in fifth. Overall I would like a vehicle that can cruise comfortably on the freeway and have decent acceleration and be able to creep around rocks and dirt in low range at a fairly slow rate.
Being the “do now think later” kind of person that I am, I immediately found a good deal on 4.56:1 gear set for my Dana 30 front axle on eBay and snatched it up, figuring I could find a similar deal on the same gear set for the rear. At the time my thinking (or lack there of) was lower is better, and I had heard other 4×4 guys throw this number (4.56) around a lot. Only after the gears arrived did I start wondering what type of rpm my motor would be spinning at freeway speed with these gears installed. This is when I discovered a great internet device called the gear calculator.
There are several different gear calculators on the internet but any of the good ones will let you plug in variables like axle gearing, transfer case gearing, transmission gearing and tire size in order to calculate your crawl ratio, freeway rpm etc.. This is an incredibly useful tool in helping you figure out your ideal gearing. With my current setup, 31″ tires, 2.73:1 rear end gears, .86:1 5th gear my jeep turns 1,654 rpm at 65mph. In 4th gear at 65mph it turns 1,923 rpm. To get a little more freeway pep I would like my motor to be spinning closer to 2,000 rpm in 5th gear, in other words similar rpm to what I’m seeing in 4th gear now. But I also plan on jumping to larger diameter tires, 33 inchers, which means I have to factor this into my regearing.
Here’s a link to a great gear calculator: http://www.grimmjeeper.com/gears.html
After plugging all the numbers into the gear calculator I find that 4.56:1 gears are in fact a bit too low. They would put my engine rpm just over 2,500 at 65mph. But 4.10:1 gears would be right in the neighborhood of where I want to be, roughly 2,300 rpm at 65mph. Still a little on the low side, but definitely livable. My overall crawl ratio, which is the lowest possible combination of gearing (low range in first gear), will also drop dramatically from its current 28.71:1 to a much lower 43.13:1. This should make difficult off-road climbs and descents a bit easier to tackle adding some real 4×4 prowess to the old CJ.
Steep and rocky ascents are much easier to conquer with the added benefit of lower gearing.
If my Jeep wasn’t a daily driver I would consider going even lower with the gears, but since it sees a lot of freeway time it looks like 4.10 gears are the way to go. Be sure and do your research before you spend big bucks on regearing. Things like tires, transmissions and how you use your truck will drastically change what the gears are ideal for your setup. Be smart and use the gear calculator first and you will thank yourself later. If not you may end up like me; anyone want to buy a brand new set of Dana 30 4.56 gears? I’ll cut you a good deal on them.
-Bill Mertz
Jeep, 4×4, ring and pinion, gears, gearing, regearing, 4.56, 2.73, 4.10, off road, CJ7
