Manual Drive Train & Axles
Work Example 13

WorkKey Level: 7
NATEF Automotive Tasks:

Problem:

How fast will a car be traveling at 3000 rpm in

a) 3rd gear?
b) 4th gear?
c) 5th gear?

Given:

3rd gear ratio: 4th gear ratio: 5th gear ratio: Rear axle: Tire diameter: 1 mile:

1.25 : 1 1 : 1 0.7 : 1 3.73 : 1 25 inches 5280 feet

Solution:

Engine-to-transmission ratio is 1.25 : 1.

So, , 3000 = 1.25 t, or = t = 2400 rpm transmission.

Transmission-to-rear axle ratio is 3.73 : 1.

So, , 2400 = 3.73 RA or = RA = 643 rpm rear axle.

Revolutions/hr: 643 rpm × 60min/hr = 38,580 rev/hr

Tire circumference: 25 inches × 3.14 = 78.5 inches

Speed: 38,580 rev/hr × 78.5 = 3,028,530 inches/hr

Speed: 3,028,530 inches/hr ÷ 12 in/ft = 252,377.50 ft/hr

Speed: 252,377.50 ft/hr ÷ 5280 ft/mile = 47.8 miles/hr
4th Gear: 59.8 mph
5th Gear: 85.4 mph