In the relationship between velocity and force when working with gears

There is no direct relationship between velocity and force. In a friction-free environment, no force is required to maintain velocity. Force is required to increase or decrease velocity (accelerate or decelerate), but as soon as the force is removed, velocity stabilizes at the new value.
When working with gears, force is multiplied according to the gear ratio of the gears (mechanical advantage). If you apply 100 lb-ft of force on the input gear of a gearset that has a 3.5 to 1 ratio, you will get 350 lb-ft of torque from the output gear (friction disregarded). This torque (force) is applied to overcome the inertia of whatever load you are applying the force to.
In practical terms, the torque of the engine, without being 'geared down', is inadequate to overcome the inertia of a vehicle (accelerate) quickly. So you start in a lower gear (higher ratio), and transition to higher gears (lower ratios) as vehicle velocity is attained. This is done manually or automatically. On most current vehicles, an overdrive gear (ratio less than 1 to 1) slows the engine speed even further for better fuel economy.
At a constant (cruising) velocity, you only need enough force from the engine to overcome wind resistance, tire resistance on the road, and the friction of all moving parts of the powertrain. So even at a high velocity (100 kph/60 mph), relatively little force is required.