The gear ratio is just the relationship between the number of teeth on the pinion compared to the number of teeth on the spur gear. So if you have a spur gear with 80 teeth and a pinion with 20 you have a 80/20 (eighty to twenty). But of course you can simplify that ratio. To do this you divide the spur gear tooth number by the pinion gear tooth number. So 80 divided by 20 equals four so the ratio is 4/1. That means that about every four times the pinion goes around the spur goes around once.
Now that you hopefully understand that gibberish let's get to the fun part. Okay, so if you increase the number of teeth on the pinion your car will have a higher top speed but less acceleration. So that means if you leave the pinion the same but decrease the number of teeth on the spur you will get the same result. Your probably wondering why you would get the exact same outcome. The reason is that in both of these cases your increasing the gear ratio. For example if I had an 80 tooth spur and a 20 tooth pinion my ratio would be 4/1. Now if I increase the pinion by five teeth and leave the spur alone (80/25) the ratio would be 3.2/1 (which is a higher ratio than 4/1). If I decrease the spur gear by 5 and leave the pinion alone the ratio would be 3.75/1. That's not exactly the same ratio but you'll get the same result of a higher top speed and less acceleration.
So if lowering the gear ratio gives the car a higher top speed and less acceleration then raising the gear ratio will have the opposite affect which is better acceleration but a lower top speed. To decrease the gear ratio you can either switch to a pinion with less teeth or switch to a spur with less teeth.
A higher gear ratio = higher top speed, less acceleration
(to get a higher ratio move to a pinion or spur gear with more teeth)
A lower gear ratio = more acceleration, less top speed
(to get a lower ratio move to a pinion or spur gear with less teeth)