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| TECH CENTER - Tech Discussion - About Mile per Hour and Revolutions per Minute
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TECH DISCUSSION
ABOUT MILE PER HOUR AND REVOLUTIONS PER MINUTE
First find the vehicle speed, MPH and the consequent engine RPM operating range:
FORMULA FOR MPH
| MPH = TIRE RADIUS ÷ 168 x ENGINE RPM ÷ GEAR RATIO |
Example: What MPH at 6500 RPM with a 4.9 rear axle and 14 inch radius
tire in 4th (1:1) gear?
MPH = 14 ÷ 168 x 6500 ÷ 4.90 ÷ 1 = 111 MPH
Example: In 3rd gear (1.34)?
MPH = 14 ÷ 168 x 6500 ÷ 4.90 ÷ 1.34 = 83 MPH
Note: Tire Radius is distance, in inches, from center of wheel to the top of the tire.
Note: Gear Ratio is Rear Axle ratio divided by Transmission Gear ratio.
FORMULA FOR RPM
| RPM = 168 x GEAR RATIO x MPH ÷ TIRE RADIUS |
Example: Using the first example, what will be the RPM after shift from 3rd to 4th
gear at 83 MPH?
RPM = 168 x 4.90 x 83 ÷ 14 = 4880 RPM
FORMULA FOR GEAR RATIO (GR)
| GEAR RATIO = TIRE RADIUS x RPM ÷ 168 ÷ MPH |
Example: Using the first example, what Gear Ratio is required for 120 MPH
at 6500 RPM?
GR = 14 x 6500 ÷ 168 ÷ 120 = 4.51
FORMULA FOR TIRE RADIUS
| TIRE RADIUS = 168 x MPH x GEAR RATIO ÷ RPM |
Example: Using the first example, what tire radius for 110 MPH but at 6000 RPM
with a 4.11 gear?
Tire Radius = 168 x 110 x 4.11 ÷ 6000
Tire Radius = 12.7 inches
Note: Approximately a 25" diameter tire. Remember that the tire radius will be less during hard acceleration than when the vehicle is standing still. Also, radius will be greater at high speed due to tire expansion from centrifugal force.
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