* Gear Ratio: A 4-inch lift often necessitates larger tires. Larger tires change the overall gear ratio, effectively making the engine work harder to reach higher speeds. If you haven't adjusted your gearing (via different axles or gears within the axles), your engine might simply be struggling to overcome the increased rotational inertia of the larger tires.
* Tire Size: What size tires are you running? The larger the tire diameter, the lower your effective final drive ratio becomes. This directly affects top speed.
* Engine Condition: A worn-out engine, particularly one with issues like low compression or poor fuel delivery, will struggle to reach higher speeds.
* Transmission Issues: Problems with the transmission (worn clutches, low fluid, etc.) could also limit your top speed.
* Aerodynamics (minor factor): While not a major contributor at 65 mph, a lift does increase wind resistance slightly.
* Incorrect speedometer calibration: It's possible your speedometer is inaccurate and reading lower than your actual speed.
To diagnose the problem:
1. Check your tire size: Compare your current tire size to the original size for your Jeep. A significant increase will drastically affect top speed.
2. Inspect your engine for issues: Look for signs of wear, leaks, or poor performance. A mechanic could perform a compression test and other diagnostics.
3. Check your transmission fluid and condition: Low fluid or damaged components can severely impact performance.
4. Consider a re-gearing: If the tire size is significantly larger than stock, you'll likely need to re-gear your axles to restore optimal performance and prevent undue stress on the engine and drivetrain. This is a common modification for lifted Jeeps.
5. GPS Speed Check: Use a GPS device to verify your actual speed to rule out a speedometer malfunction.
If you're unsure about any of these steps, it's best to consult a mechanic specializing in Jeeps or off-road vehicles. They can properly diagnose the issue and recommend the best course of action.
