Originally Posted by Jerry Bransford
For 33" tires and the 5-speed, you want 4.56. 4.10 would give you the same rpms as how the Jeep was delivered with 30" tires and 3.73 gearing but with bigger tires and a lift, you actually need the slightly higher rpms of 4.56 to truly restore the power lost by installing larger tires.
The bigger tires and suspension lift mean slightly higher rpms are required at the same speeds to maintain factory performance. The bigger tires are heavier, are harder to turn, have more rolling resistance, and have more leverage against the engine. The bigger tires also combine with your suspension lift to raise the Jeep higher into the windstream for more wind resistance which is just one more thing to require slightly higher than factory rpms to compensate for.
In other words, don't fear slightly higher rpms as some seem to be afraid of. 4.10 gearing with 33" tires and the 5-speed will just have you downshifting to 4th gear more for even slight grades on the highway, giving you higher rpms than even 4.56 would in 5th gear would.
Don't fear the gear, most beginners under-gear their Jeeps because they think their Jeeps should have car-like engine rpms which is not realistic for a Jeep with big tires and the aerodynamics of a barn door.
Cost-wise, $1100-1200 is pretty typical for most areas. Don't try to save a few $$$ by buying the gears on your own, have the installing shop provide all parts or you could be in for a round of finger-pointing between the shop and where you bought the parts from if anything goes wrong during or after the installation. Having the installer provide all parts will make handling any labor or parts warranty issues a snap.
Jerry is absolutely right. Many people think their fuel mileage will suffer by going with shorter gears. This is true only at extremes; in fact, the opposite is actually true. With tall gears, your mileage will suffer in town and on hills, as it takes much more power to get the vehicle moving (similar to taking off on a ten-speed bike in the wrong gear). I install a lot of gears and I always tell people to go at least one size lower (numerically higher) than they think they need. They never listen and almost all of them tell me later that they wish they had. Even with very short gears, your transmission's overdrive is more than capable of handling freeway speeds with no problem.
Also, the gear charts are very misleading. They are more accurate for a 5-speed than they are for an auto though, as the auto will need even lower gearing (numerically higher). Like Jerry said though, larger tires have more weight. The charts take into consideration the diameter of the tires, but they don't consider width, contact patch, weight, rolling resistance, etc. A decent way to determine your gear ratio is to use the following equation:
New tire size divided by old tire size times old gear ratio = new gear ratio. If you are in between gear ratio sizes, ALWAYS round up (not down) to the numerically higher gear size and then jump up one more size (possibly two depending on your engine power, weight of accessories, if you live in the mountains, etc.).
My XJ with an AW4 auto came stock with 3.55 gears. I initially re-geared to 4.56 for 35's, but the vehicle was constantly shifting and searching for gears on even moderate inclines. I have a set of 4.88's that I plan to install very soon. I would have gone with 5.13's, but they are not available for the Dana-30. I drive this vehicle on the freeway at 80 all the time, and the 4.56's are way to tall. 35/ 29 x 3.55 = 4.28. Rounding up takes you to 4.56 (which was too tall on my vehicle), and jumping one size gets to 4.88's, which should will be better, but 5.13's would give much better power and still give near stock fuel economy, even at highway speeds. Too bad they're not available for the Dana-30...
My YJ with an AX-15 5-speed came stock with 3.07 gears. I just installed a set of 4.56 gears to run 35's (mainly because I had a set sitting in the shop along with lockers, so I used them to save money). This will be much better than on the XJ because of the lower gearing of the 5-speed (second gear in the AX-15 is almost identical to the first gear of the AW-4), not to mention the better control you have over the shift points with the manual trans. Now my YJ at 65 MPH in 4th gear runs 2395 RPM's (1 to 1 ratio and 65 MPH is the standard for the gear chart I am using(Tire Gear Ratio Charts
). The chart says that in order to get that same 2395 RPM's with 31-inch tires, I would need 3.42 gears. If that is correct, then why does Jeep use 4.11's on the Rubicon models? 35/29 x 3.07 = 3.71 (consider that the same as 3.73). Round up to 4.11 and jump one size gets you to 4.27. In my case, I went to 4.56's, which should be fine due to the extra width, weight and resistance of the 35x12.5x15's, and I am very confident that the highway fuel economy will still be great.
My LJ Rubicon with the 42RLE auto trans came stock with 4.11's and 31-inch tires. It is now on 35's and I will be installing 5.13's to make it right. This is my daily driver (and I put a lot of miles on it) and I could buy any size gears I want for it. I am buying 5.13's because this will get it back to stock ( I don't want to go too crazy since it is my daily driver, but my mileage is suffering with the big tires and relatively tall gearing). 35/31 x 4.11 = 4.64. Round up to 4.88 and then jump up one size and you are at 5.13.
In your case, 33/29 x 3.07 = 3.49. Round up to 3.55 and jump one size and you are at 3.73. This is a great example of the charts not being accurate, nor is this formula 100%. Personally, I would use either 4.11's or 4.56's. Jeep uses 4.11's for 31's on their TJ Rubicons (and 32's on their JK Rubicons). I installed a set of 4.11's on a friend's YJ with 31's and he loves it. He drives it well above normal freeway speeds and has no problems with fuel efficiency. Your 33's will be 2-inches wider than his 31's and therefore have a greater contact patch and greater rolling resistance (more rubber on the ground); so like Jerry recommended, 4.56's should give you great power and your fuel economy will be fine.