One of the first upgrades that a Jeep owner will install is a lift kit and tires. Of course there’s the occasional stock Jeep with every other add-on found in a 4WP catalogue besides the lift and tires. But that’s really the exception, and for good reason. A Jeep without a lift kit and large, oversized tires is like a two-wheel drive pickup truck (or a two wheel drive JKU). There’s just so much more the Jeep is capable of, you wonder why it didn’t come lifted from the factory. However, after throwing on a lift kit and large tires, many owners are surprised when the performance of their newly modified vehicle seems lacking. That’s because it is. When you add larger wheels and tires to any vehicle, especially a newer one, it throws off a variety of systems. To a certain extent, lifting a vehicle does increase the profile and decreases the aerodynamics, but the real kicker are the wheels and tires. Besides, a Jeep isn’t aerodynamic anyway, especially the classic Wranglers and the older CJ’s. They’re practically a brick on wheels. So how do you get your lost power back without an engine swap? A re-gear, of course. On older straight-drive vehicles, you can simply shift higher in the RPM range when the tires are a few sizes over OEM. But if you go too large or if you have an automatic transmission, you’ll need a re-gear. When you add larger tires, the computer shift points on an automatic transmission will be off, the vehicle will feel sluggish, and the engine will not operate inside the power band. With a stick shift, you can force it to operate inside the power band to a certain extent. But acceleration will suffer, and the power bands will no longer overlap between shift points. In other words, with a stick shift you’ll need to over-rev the engine to shift in order to not lug the engine when upshifting. With an automatic, the computer will over-rev or “hunt” for the proper gear at odd times. It will also lug the engine once it shifts or begins cruising. Lugging happens when the engine operates in a lower RPM range for a given gear. When you re-gear, you change the gears inside the front and rear axle/differentials, to accommodate the larger wheels/tires. This brings the engine operating range and power band back into spec roughly as it was from the factory. If you’re adding larger tires, you’ll add lower gears. Lower gears are numerically higher. For example, an early model stick shift Jeep Wrangler TJ 2.5L features 4.10 gears (inside the differentials) from the factory; factory tire size was about 29” in 1997. When upgrading to a 35” tire, the stock gears are insufficient, as the tire is now much larger than stock. The resulting combination would be poor acceleration, low highway speeds, and inability to pass anything smaller than an 18-wheeler on road (and even then, that’s questionable). To shift, the engine would need to be over-revved and operated on the high end of the power band, in order to be driven on-road. This results in poor fuel economy and a vehicle that isn’t much fun to drive. To fix this, the axle ratio is changed from 4.10’s to 4.88’s. This allows the engine to “pull” through the gears more effectively, operate inside the power band, and will restore “lost” top-end gears, such as 5th or 6th gear overdrive. Fuel economy is restored to about OEM spec, and the Jeep powertrain operates in the manner it was intended from the factory. With automatic transmissions, a re-gear will fix these problems, such as sluggish acceleration, poor highway speeds, and odd shift points. With newer vehicles like the JK Wrangler, an AEV ProCal is required to adjust the computer after re-gear. While a re-gear may seem like a daunting or expensive task, just ask anyone who has upgraded from OEM gears after running larger-than-stock tires and they’ll tell you it’s worth it, for good reason. If you’re unsure of how to interpret the many re-gear charts, or you’re curious as to what gears your Jeep needs, just let us know - we’re more than happy to assist.