Jeep Wrangler TJ Dana 35 Axle Rebuild
Gas mileage is an issue for Wrangler owners, especially those with modifications. In fact for every 100lbs you add to your vehicle, you’ll lose around 2% MPG (fueleconomy.gov). Throw on some bumpers, a tire carrier, winch and accessories, and your ride will be several hundred pounds heavier. Then factor in the additional drag caused by the lift, and don’t forget that your stock gears are struggling to turn all this over. Before you know it your Wrangler is averaging 11mpg. This is unacceptable, especially when it’s a daily driver.
The most practical modification to increase fuel efficiency on a build is to re-gear the axles. Granted every vehicle is different, but for our BRC-12 build re-gearing was the best solution. Ideally, your gears, transfer case, transmission, and engine should sing in harmony. Just because a four-cylinder engine is smaller than a six or eight-cylinder, doesn’t mean it’s more efficient – especially for Jeeps. That’s because a four-cylinder can overwork itself to compensate for the lack of power. This negates all benefits of fuel efficiency.
Now that we have proper power (in this case an I6 4.0L), we must find the weak links in the drivetrain. Fortunately the ’99 Wrangler comes standard with an AX15 Transmission and NVG231 transfercase, and both are quite capable. This leaves the factory 4.10 gears which can handle the 33″x12.50″ Mickey Thompson Baja MTZ‘s just fine, but aren’t necessarily the best to crawl with.
Ring & Pinion
So we have to find an ideal gear ratio that’s effective on the trail, and efficient on the highway. To do this we need to consider the axles, transfercase, and transmission ratios as well as the tire diameter. We want to keep the highway RPM’s around 2700 for fuel efficiency. We calculated (Speed Potential Calculator) that 4.56 gears would be the best for our build – this ratio cruises well at 75mph, and gives the BRC-12 the ability to crawl when necessary.
The crawl ratio is another factor in this process. Crawl ratio is the lowest gear ratio the vehicle is capable of. It considers the ratios of the transmission, transfer case, and axles – (transmission first gear ratio x transfer case low gear x axle gear = crawl ratio). So the TJ’s crawl ratio is approximately 48 (3.83 x 2.72 x 4.56 = 47.5), which is acceptable for the purposes of this build since we’re MPG conscious. The higher the ratio, the better the crawling ability. A ratio in the 60’s is pretty capable, but i’d need 5.71 gears or a different transfer case/transmission. This ratio would drastically hurt my highway efficiency which is unsuitable for a daily driver.
We went with Superior Axle & Gear for all the necessary parts for the axle rebuild. Their ring and pinions are manufactured with premium 8620 steel, and are cut using the finest Gleason and Phoenix gear cutting machinery. Each piece is heat treated, honed, and lapped for ideal contact patterns.
This allows for a minimal break-in period for Superior Axle & Gear ring and pinions. We ran the gears through half a dozen heat cycles to completely harden them, and then changed the oil to flush any residual shavings.
But stronger gears aren’t enough if you plan on rock crawling with confidence; especially with a Dana 35 rear end.
We replaced the stock 27 spline axle shafts with Superior Axle & Gear Super 30 and Super 35 axle shafts. Not only are these shafts a stronger alloy, but they’re larger in diameter with a 30 spline count (JeepForum). This allows more points of contact which in turn makes them stronger. In particular the Super 35 was necessary because it eliminates the c-clip which is notorious for failing. With these axles we’ll be able to crawl and torque the lug nuts without worry. Plus, they have the strength to turn over larger tires if we choose to do so.
In all reality, a Dana 44 or 60 axle swap is a bit overkill for a daily driver on a budget. That’s why we decided to install a Superior Axle & Gear rear axle truss for the stock Dana 35. This is the perfect addition for those who don’t have the time or money to change an entire axle assembly on their Jeep TJ. According to Superior Axle & Gear, this truss provides an additional 52% horizontal strength, and can be installed in about four hours.
Not only does this rear axle truss give the Dana 35 additional strength and durability, but it’s also compatible with most bolt on suspension systems. There were no complications with the 4″ Skyjacker Suspensions lift we used on our TJ. It simply bolts directly onto the Dana 35 and the track bar relocation bracket. The shocks have to be remounted to the truss itself, but don’t have to be pulled out any further.
It’s time to add functionality now that the axles are rebuilt for strength; because no offroad machine should leave home unprepared. Next week we’ll talk about adding our air compressor and a set of lockers to the front and year.