**1. Why Gears?**

Imagine a car whose engine is directly connected to its drive wheels -- every revolution of the engine turns the drive wheels once. That is, 1 engine revolution per minute (RPM) results in 1 drive wheel revolution per minute.

Let’s consider how fast will that car will go when its engine runs at 1,000 RPM.

Wheels with 165-70/14 tires have a circumference of 72.55 inches, which should be close to my 1974 Spider’s original stock tire size. With the engine running at 1,000 RPM those wheels will be revolving at 1,000 RPM. We can calculate how fast those wheels are propelling the car by

Code:

```
1,000 revolutions 72.55 inches 72,550 inches
----------------- * ------------ = -------------
minute revolution minute
```

Code:

```
72,550 inches 60 minutes 1 foot 1 mile
------------- * ---------- * --------- * ---------- = 68.72 MPH
minute hour 12 inches 5,280 feet
-----inches per hour-----|
-------------feet per hour-----------|
------------------miles per hour------------------|
```

(Aside: Some of you may have calculated an approximation in your head: 1,000 RPM * 6 feet per wheel revolution is 6,000 feet per minute, which is slightly over 1 mile per minute or 60+ MPH.)

What happens as engine RPM increases? In this simplified model any increase in engine RPM will result in a corresponding increase in wheel speed. At 4,000 engine RPM the wheels will be spinning 4 times as fast as at 1,000 RPM and the car will now be moving at 68.72 * 4 = 274.8 MPH. Tires surely smoking, probably shredded.

Ummm, I don’t think so! These speeds are clearly not realistic.

Thus, it's easy to conclude that engine RPM must be reduced to a lower, more manageable number of drive wheel revolutions per minute.

**Note**Other calculations will also need to convert from

*inches per minute*to

*miles per hour*, so here is the multiplicative conversion factor that does so:

Code:

```
60 minutes 1 foot 1 mile minutes miles
---------- * ------------ * ---------- = 0.0009469 -------------
hour 12 inches 5,280 feet hour inches
```