This page will show any calculations I work thru for this project. I encourage comments as I may miss things and want to be accurate.

## Motor RPM

My truck has no tachometer. I estimated the rpm at 45 mph was around 2000 but wanted to verify it with calculations. I want to figure current draw to the motor to size the battery capacity. This means I need the motor rpm at an average speed. I started with assuming a **45 miles per hour average speed**.

So I wanted to measure the tire diameter and calculate the circumference- the distance traveled per tire revolution. Tire diameter = 27 inches. The circumference is PI (3.14159) times the diameter or 84.82 inches per revolution.

In order to see how many rpm the tire goes, I converted 45 mph to feet per minute.

45 mph times 5280 feet per mile = 237,600 feet per hour, then divided by 60 minutes per hour to get 3960 feet per minute.

To calculate how many tire revolutions per minute, I divided 84.82 inches circumference by 12 inches per foot to get 7.1 feet per tire revolution.

Now to figure the tire revs at 45 mph (3960 feet per minute), I divided 3960 by 7.1 to get **560 tire rotations per minute** to achieve 45 mph speed.

OK! Deep breath now. I knew how fast the tire spins at 45 mph. Now, How fast does the engine spin? I found that the rear end ratio for my truck configuration is 3.73. That means, for every rotation of the tire, the drive shaft rotates 3.73 times. So at 45 mph (3960 fpm) the **drive shaft rotates 560 times by 3.73 or 2089 rpm**.

ButÂ what about the engine? The fifth gear ration for this transmission is 0.83. So at 45 mph, the engine must be spinning at 2089 times 0.83 or **1734 rpm**.