* Modified Converter: The modified stall converter is the most likely culprit affecting the behavior at 55 mph. A higher stall speed converter means the engine revs higher before the transmission engages fully. This can lead to a smoother transition at lower speeds but might cause the engine to be at a lower RPM at highway speeds (like 55 mph) than a stock converter. At 55 mph, the engine might be comfortably settled into a lower RPM range where noise and vibrations are minimized. This is a "quieter" condition, not necessarily a problem.
* Gear Ratio: The rear axle gear ratio plays a crucial role. A lower numerical gear ratio (e.g., 3.50:1) will result in lower engine RPM at higher speeds compared to a higher ratio (e.g., 4.10:1). A lower ratio is more suited for highway driving and better fuel economy, resulting in quieter operation at 55 mph.
* Engine Condition and Tune: A well-maintained 400 cubic inch engine, properly tuned, will naturally run quieter than one that's neglected or poorly tuned. Factors like ignition timing, valve adjustment, and carburetor settings all impact noise levels. At 55 mph, it's already operating at a lower RPM where some engine noises become less pronounced.
* Transmission Condition: While less likely to be the primary cause of *quietness*, a well-maintained transmission contributes to a smoother, quieter operation. If it were experiencing problems, that would likely manifest as noise or issues at various speeds, not just quiet operation at a specific speed.
* Aerodynamics (less likely): At 55 mph, aerodynamic noise is relatively low. It's unlikely to be the key factor explaining a *quiet* operation.
In short: The quietness at 55 mph is probably a consequence of the modified stall converter working in conjunction with the rear axle gear ratio to put the engine in a sweet spot of RPM where it's efficient and quiet. Unless there are other accompanying symptoms (like poor acceleration, overheating, strange noises at other speeds, etc.), it's likely not a problem. However, if other issues are present, these could be contributing factors.
It's important to note: "Quiet" is subjective. If the truck was previously noisy and now is quiet at 55mph, that's a change that warrants attention to see if it's related to a needed repair or simply a change in driving characteristics. If it's always been quiet at that speed, then it's likely perfectly normal operation.
