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Southern Performance Systems
Phone: 678-482-0866
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Today's Muscle Engines for Yesterday's
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Site Updated: 8/28/17

For the Example we will be use a 285/70R/15 Tire.
285 = Tire Size
70 = Aspect Ratio (Must use a decimal so 70 would actually be 0.70)
15 = Wheel Diameter

Tire Diameter (Tire Height) = Tire Size / 25.4 x Aspect Ratio x 2 + Wheel Diameter
Example: 285 / 25.4 x 0.70 x 2 + 15 = 30.70 Tire Dia.

Tire Circumference (Inches) = 3.14159 x Tire Diameter (Inches)
Example: 3.14159 x 30.70 = 96.4467" Tire Cir.

Tire Circumference (Feet) = Tire Circumference (Inches) / 12
Example: 96.4467 / 12 = 8.037' Tire Cir.

Tire Revolutions Per Mile = Number of Feet in a Mile / Tire Circumference (Feet)
Example: 5,280 / 8.037 = 656.9616 Tire Rev. Per Mile

Rear End Gear Ratio = Engine RPM @ Cruising Speed / Tire Revolutions Per Mile
(Note that Cruise Speed is between 60-65 MPH)
Example 1 (without Overdrive): 1800 / 656.9616 = 2.74
Example 2 (without Overdrive): 2000 / 656.9616 = 3.04
Example 3 (with Overdrive 30%): (1800 / 656.9616) x 1.3 = 3.55
Example 4 (with Overdrive 30%): (2000 / 656.9616) x 1.3 = 3.95

MPH = (RPM x Tire Circumference (Feet)) / (Trans. Gear Ratio (5th) x Final Drive Gear x 88)
Example: (2000 x 8.037) / (0.79 x 3.04 x 88) = 76 MPH (Rounded Figure)

RPM = (MPH x Trans. Gear Ratio (5th) x Final Drive Gear x 88) / Tire Circumference (Feet)
Example: 76 x 0.76 x 3.04 x 88) / 8.037 = 1,922

Displacement in Cubic Inches = Bore (In) x Bore (In) x Stroke (In) x 0.78545 x # of Cylinders
Example: 4.060 x 4.060 x 3.620 x 0.78545 x 8 = 374.9464ci

Compression Ratio
CR = (V1 + V2) / V2
CR = Compression Ratio
V1 = Cylinder Volume at Exhaust Closing
V2 = Combustion Chamber Volume

Common Conversions
Miles to Kilometers: Miles x 1.609344
Liters to Cubic Inches: Liters x 61.023744
Inches to millimeters: Inches x 54.4
Millimeter to Inches: Millimeters x 0.0393701
80cc = 0.08 Liters
1ci = 16.39cc
Mean Piston Speed in Feet per Minute
PS = 0.166 x Stroke (in Inches) x RPM (crankshaft speed)