# Electric RC Theory 101: Calculating Gear Ratios, Rollout, and Top Speed

For this example I will be taking my Hot Bodies D8T Monster Truggy as an example to base the calculations on. Here are some details

• Motor: Castle Creations 2,200kV model 1515 1Y
• Pro-Line 3.8" Badlands with an Outer Diameter (OD) of 148.33mm tyres.

The gear ratio at the centre differential

```50T Spur/13T Pinion = 3.85:1 gear ratio
```

For the front and rear diffs, the axle ratio is

```Beveled spur 43T / Spiral pinion 10T = 4.3:1
```

We can therefore calculate the Final Drive Ratio (FDR)

```FDR = (3.85)(4.3) = 16.6:1
```

Let's now take a look at rollout which is defined as

Rollout is the ratio of how many revolutions of the motor causes the tires to move one inch.

and `1" = 25.4mm`

```466mm/16.6 = 28mm per motor rev.
28mm/25.4mm = 1.1:1 rollout
```

I typically run this @ 4S which means `2200kV x 14.8vdc x 0.9 = 29,304 RPM`; note that I've compensated for a 10% drop in cell voltage due motor loading, i.e. `4.2v/cell x 0.9 = 3.78v/cell` @ storage charged Lipo.

We can therefore assume the truggy would be able to reach a top-speed of 49.2 km/h (30.6 mph)

```29,304 rev/min x 28 mm = 0.82 km/min x 60 min/hour = 49.2 km/h
``` 