Changing tires does not really change the gears. High gears (a low number) will let the car cruise at high speed but at low rpm. It also makes the car harder to accelerate. Low gears (a high number) makes the car run more RPM at high speed, but lets the car accelerate faster. Imagine a 10 speed bicycle, but let's look only at first and 10th. Low gears is like first gear. The pedals go around as fast as possible, but the bike does not move very fast. Easy to accelerate and climb hills, but top speed is not an option. Now shift to 10th. It takes all your muscles and still you accelerate rather slowly, but top speed is whatever your body is capable of. That's the way it works in a car, too. The same engine will reaact very differently depending on the gears.
