# Seeing Gear Ratios by Listening

Sometime around my tenth birthday, my dad bought a car with an RPM gauge. Up until that time, I knew that the speed of the car was related to vibration of the engine, but I didn’t know how to measure it. An engine has an optimal operating range that is matched to the wheels by a transmission. The transmission changes the gear ratios up or down depending on the desired speed of the engine or the power necessary to accelerate the vehicle. All of these things operate as a system.

I wanted to understand the relationship between RPM, the pitch of the engine sound, and miles per hour so I would build models in my imagination by watching the various gauges and listening to the engine. I could hear that the engine sounds were related by ratios and numbers. In my imagination, I could “see” the scale between the sounds, based on the pitch, so I had a very rough idea of the various gear ratios between first and second gear, and second and third gear. (I never did figure out reverse…that one still bugs me!)

This is when I first started to visualize percentages and ratios as gears. Gear ratios can tell you the relative relationships between individual parts in a system. It’s easy to find the relative relationships between certain things if you think in terms of gear area and angular velocity. This observations can come in handy when you deal with a lot of numbers that change over time.