There isn't a "better" combination in the sense of a significant performance upgrade easily swappable *without* major modifications. Finding a different transmission that directly bolts up to both your engine and the NP231 is highly unlikely. Any alternative would require significant custom work (adapter plates, driveshaft modifications, etc.), making it a complex and expensive undertaking.
Therefore, focusing on improving your *existing* AX-15 transmission (assuming that's the correct designation) or the overall drivetrain is likely a more cost-effective and practical approach. Consider these options instead:
* AX-15 rebuild/upgrade: If your AX-15 is worn, a rebuild with stronger internal components can significantly improve its reliability and lifespan. This is a common upgrade for these transmissions.
* Gear ratio changes: If you're unhappy with your vehicle's performance (e.g., poor acceleration or high RPM at highway speeds), consider changing your axle gear ratio. This is a common modification to improve performance or fuel economy depending on your needs and driving style.
* Clutch upgrade: A stronger clutch will better handle higher torque outputs if you make other performance modifications to the engine.
In summary, swapping the transmission is a complex and expensive proposition unlikely to yield significant benefit over improving what you currently have. Focus on upgrading or maintaining the components you already have for a more realistic and cost-effective approach to improve your Jeep's performance and reliability.
