The crank shaft of a car is rotated by the engine at a speed depending on how much throttle is applied, measured in rotations per minute (rpm). A typical value is 3000rpm. Gears in a car change the rpm of the crankshaft to turn the wheels at different rates so that the car can move at varying speeds.
The downside of this is that you have less driving force (torque), which is why it’s easier to accelerate using lower gears. It’s a bit like riding a bike, where it takes less effort to go up a hill in low gear, but you don’t go as far.
The Ice Cold In Alex ambulance is hand cranked, in reverse gear, up a sand dune - a very low gear. So here are some rough calculations for what the gearing ratio might be.First, let’s assume the wheel is half a metre in diameter - just how far will it travel per turn? 
For a speed of 8000 metres (80km) per hour the wheel makes: 
Which is 6349/60 = 105 revolutions per minute. The gearing ratio is therefore: 
Twenty nine it is, then.
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