* Gear Ratio: Fifth gear in a Wrangler is an overdrive gear. This means it's a higher gear ratio than 1:1. While it reduces engine RPM at highway speeds, it doesn't necessarily mean a *significant* increase in speed compared to fourth gear, especially if you're already at or near the engine's peak power output. The speed increase might be marginal.
* Engine RPM: If your engine is already close to its maximum RPM in fourth gear, shifting to fifth won't yield a substantial speed increase. The engine simply doesn't have the power to accelerate further at that gear ratio.
* Aerodynamic Drag: At higher speeds, aerodynamic drag becomes a significant factor. The engine may not be producing enough power to overcome this drag and accelerate further in fifth gear.
* Load: A heavier load (passengers, cargo, towing) will reduce acceleration in all gears, making the difference between fourth and fifth gear less noticeable.
* Terrain: Going uphill, even slightly, will significantly impact your top speed in any gear, especially fifth.
* Mechanical Issues: There's always a chance of a mechanical problem:
* Transmission Problems: A problem with the fifth gear itself (worn synchros, damaged gears) could prevent it from engaging properly or cause slippage.
* Drivetrain Issues: Issues with the differential, axles, or driveshafts could limit speed.
* Tire Size: Larger than stock tires will change your speedometer reading and effective gear ratios. This could make the speed increase from fourth to fifth seem less significant than expected.
To troubleshoot:
1. Check your speed in fourth gear at a safe location: Make sure you're not already close to the maximum speed your engine and vehicle can achieve.
2. Observe the engine RPM: Note the RPM in fourth and fifth gear at the same speed. A significant drop in RPM in fifth gear indicates it's functioning correctly. If the RPM remains high, it might suggest a mechanical issue.
3. Check for any other drivetrain noises or issues: unusual sounds or vibrations can indicate underlying problems.
If you've ruled out the more common reasons (load, terrain, RPM) and still experience this issue, it's best to have a mechanic diagnose potential mechanical problems with your transmission or drivetrain.
