Must you change the gears in truck with lif t and bigger tires?

Generally, yes, you should change the gears (gear ratio) in a truck with a lift and bigger tires. Larger tires increase the overall diameter of the wheel, effectively increasing the distance the truck travels per engine revolution. This means the engine will work harder to achieve the same speed, leading to:

* Reduced power and acceleration: The truck will feel sluggish and less responsive.

* Worse fuel economy: The engine will have to work harder, consuming more fuel.

* Higher engine RPMs at highway speeds: This can lead to increased engine wear and noise.

Changing the gear ratio (usually by swapping out the ring and pinion gears in the differential) effectively "recalibrates" the drivetrain to compensate for the larger tires. A lower gear ratio will restore the balance, allowing the engine to operate in its optimal RPM range for various speeds.

While some minor lift kits and tire size increases might be manageable without a gear change, substantial lifts (3 inches or more) and significant tire size increases almost always necessitate a gear ratio change for optimal performance and longevity. Failing to do so will negatively impact the truck's drivability and reliability.