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Glossary of Terms & Calculations
The following is list of terms and calculations frequently used to describe vehicles and their conponents.
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ENGINE
Displacement (cu.in./mm): Total cylinder volume, Calculated in cubic inches using the formula
.7853982 x bore² x stroke x number of cylinders.
To convert to cubic centimeters (cc), use the conversion factor of 2.54 for bore and stroke.
.7853982 x (bore x 2.54)² x (stroke x 2.54) x number of cylinders.
Bore x stroke (in.): Diameter of cylinder bore and length of piston stroke.
Compression ratio (:1): The relationship between the combined cylinder and combustion chamber volumes with
the piston at Top Dead Center (TDC), and the combustion chamber volume (alone) with the piston at TDC.
Calculated using the formula:
Cylinder vol. +chamber vol.
Chamber volume
Combustion chamber volume, measure in cubic centimeters, is calculated using a burette marked in cc’s and filled
with light-colored liquid: oil, solvent, or even water (The procedure is known as cc-ing). With the engine assembled,
piston at TDC, valves closed and spark plug removed, the liquid is poured from the burette into the plug hole until
it reaches the beginning of the plug threads. Any deck height, head, and gasket thickness measurements must also be
added (again converted to cc’s using the 2.54 conversion factor), if applicable, to arrive at total chamber volume.
If you know your vehicle’s compression ratio already, combustion chamber volume can be calculated thus:
Total cylinder volume (cc)
Compression ratio – 1.00
The higher the compression ratio, the more air-fuel mix will be compressed and the more powerful the combustion.
As compression ratio rises, so does the need for higher-octane gasoline.
Fuel Injection: Electronically controlled fuel induction, either single-point (cheaper, less complex, less precise)
or multi-point (more complex more precise and costlier).
Valve gear: Engine valve-train design is either overhead camshaft (the case with all imported trucks and a few domestics)
or overhead valves (the traditional design among domestic truck engines). Overhead cam engines locate either a single cam
(SOHC) or dual cams (DOHC) in each cylinder head to directly actuate two, three or four valves per cylinder. Overhead cam
designs, normally seen in smaller engines, form a stiffer valvetrain that can spin faster make more power at higher rpm.
Overhead valve engines (OHV): utilize a single camshaft inside the engine block to actuate two valves per cylinder via
pushrods that reach all the way to the cylinder head. This design is considered fine for larger engines intended to
produce torque as a priority.
Redline (rpm): Factory-recommended limit for engine rpm.
Fuel requirement: Factory-recommended octane minimum. In some cases where the factory recommends 92 octane for a
high-performance/high-compression engine, a lower octane can be used but will reduce maximum horsepower rating.
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ENGINE OUTPUT
Max. Rear-wheel hp @ rpm: Chassis dyno results to indicate peak output at the rear wheels, and at what rpm level it occurs.
Max. rear-wheel torque (lb.-ft.) @ rpm: Chassis dyno results to indicate peak output at the rear wheels, and at what
rpm level it occurs.
Mfg.’s rating, SAE net (Hp and torque): Factory supplied; maximum torque at the flywheel with all auxiliary systems
hooked up and functional.
Brake Horsepower: A measure of power per mile found more often in European specs. Calculated:
rpm x torque (lb-ft)
5,250
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ACCELERATION
0-30 mph (sec.): Elapsed time from zero to 30 mph, measured by computerized fifth-wheel device.
A measurement of torque off the line.
0-60 mph (sec.): elapsed time from zero to 60 mph intended to replicate a vehicle’s performance entering a highway.
Standing ¼-mile (sec. @ mph): Time required to travel 1,320 feet, and vehicle speed at end of test: the industry
standard of top-end acceleration performance.
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DRIVETRAIN
Transmission ratios: The number of revolutions each gear makes for each revolution of the engine. Documents high or
low gearing, as well as gear ration spread.
Final drive ration (:1): Calculated using the formula lowest (numerically highest) transmission gear x axle gear.
Low-range ratio (:1): Gear reduction via a gear set in the transfer case. Generally speaking, the lower the low range ratio,
the better its rock-crawling ability, and the greater the vehicle’s ability to mount taller tires and still have sufficient
gearing for trail use.
Axle ratio: Number of driveshaft revolutions per revolution of the axle shafts. Can be calculated by dividing the number of
teeth on the axle ring gear by the number of teeth on the pinion gear.
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RING/PINION CALCULATOR
There are many ways to estimate what your gear ratio is. One popular method is to mark your tire and driveshaft, then slowly
roll the vehicle so the tire turns one full revolution. While you do this, count the number of turns (down to the tenth) of
the drive shaft. This will give you a close idea of your gear ratio.
Formula to used with Ring Gear and Pinion Gear
Ring Gear Teeth Count / Pinion Gear Teeth = Gear Ratio
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STEERING
Types: One of two types rack and pinion or recirculating ball, the latter more commonly used on domestic applications.
Turns, lock to lock: Number of turns of the steering wheel makes from full-lock right to full-lock left.
Turning circle (in.): With the steering wheel at full lock, the distance the vehicle can negotiate a 180-degree turn,
measured diametrically.
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BRAKES
Front and rear: Size and type of brakes. Discs are usually used in front, drums in the rear.
Swept area: Illustrates total braking area provided by all four wheels, the total area of the discs and/or drums swept
by the brake pads in a single revolution of each wheel.
Antilock: Electronically controlled, sensor-actuated braking system designed to minimize or eliminate brake locking.
RWAL (real antilock) is standard on most pickups. 4WAL (four-wheel antilock) is usually offered as a factory option,
and is sometimes standard. Four-channel antilock indicates that each wheel has its own sensor.
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BRAKING DISTANCES
30-0 mph (ft./sec.): The distance and time required to stop from 30 mph, without wheel lockup.
55-0 mph: Same as above from 55 mph. Intended as a measure of stopping at free-way speeds.
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WHEELS AND TIRES
METRIC TIRE TO DIAMETER (INCHES) CALCULATOR
This calc converts a Metric tire to inches.
Most of the formulas dealing with gear ratios will want a tire diameter (measured in inches). This formula is a quick
way to get the tire diameter of those metric tires that are common on just about everything stock. For example a
LT265/75R16 would be around 31.6 inches tall and 10 inches wide. Enter any three of the numbers into this form to solve
for the fourth. "LT" means Light Truck and "P" means Passenger tire. The bigger number (on the left) is the Section Width.
The number to the right of the slash ("/") is the Aspect Ratio (percent of width). The "R" means Radial tire and the last
number, far right, is the rim diameter in inches.
Formula to use
Width in inches = section width / 25.4
Section Height in inches = Width in inches X Aspect Ratio (%)
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ACTUAL SPEED CALCULATOR
Speedometer ratio adjustment calculator.
Ever wonder how far off your speedometer is with your new bigger tires? By using a simple ratio calculation,
this info will only approximate your corrected speed and depends on the accuracy of tire size diameters
(assuming nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.)
then use the Gear Ratio calculator, it's probably a little more accurate.
Formula to use
(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH
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FUEL ECONOMY
Epa city/highway (mpg): Miles-per-gallon (mpg) estimates provided by the EPA.
Actual, combined (mpg): Calculated mileage for duration of the road test over a wide variety of terrain,
meant to simulate "real-world" use.
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DIMENSIONS AND CAPACITIES
Wheelbase(in.): Distance from the center of the front wheel to the center of the real wheel.
Length (in.): Overall length of the vehicle from bumper to bumper.
Width (in.): The distance between the widest points on both sides of the vehicle includes body-side molding.
Height (in.): Overall height to the tallest point of the roof.
Overhang, front/rear (in.): Distance measured from the center of the front wheels to the furthest forward point,
and the center of the rear tires to the furthest rearward point. The less overhand a vehicle has, the better it will
usually negotiate uneven terrain off-highway.
Track, front/rear (in.): The distance between the centerlines of the tires, front and rear.
Mininum ground clearance (in.): The distance from the ground to the lowest point of the chassis.
Ride height (in.): The distance from the ground to the driver’s seating surface. Tells how high the driver sits affects
visibility.
Load lift height (in.): The distance from the ground to the cargo floor via an open tailgate. Tells how easy a vehicle
is to load and unload.
Max. cargo capacity (cu. ft.): The volume for cargo, with all seats folded up and out of the way (e.g. an SUV).
Approach/departure angles (degrees): Formed by drawing a line from the front tire’s contact patch to the tip of the front
bumper, and by drawing a line from the rear tire’s contact patch to the tip of the rear bumper. The shorter the overhand,
the greater the angle and the more maneuverable in uneven terrain.
Breakover angle (degrees): The angle formed by lines drawn from the front and rear tire’s contact patches and the point
on the chassis which marks the mid-point of the measured wheelbase. The taller the vehicle’s chassis, the smaller the breakover angle.
Base curb weight (lb.): Weight of the vehicle with all fluids, but no passengers and no payload, at base trim level.
Base weight distribution, front/rear (%): Comparison of front-to-rear curb weight distribution. A 50/50 balance front to
rear is considered ideal for handling.
GVWR/GCWR (lb.): Gross Vehicle Weight Rating, measured as curb weight + payload; and Gross Combined Weight Rating,
measured as curb weight _ payload + towing capacity.
Payload (lb.): The amount of passenger and cargo weight the vehicle is rated to carry.
Maximum towing capacity (lb.): The maximum weight the vehicle is rated to tow. Based on engine power, gearing, cooling
capability, rear spring rates and hitch quality, among other things.
Fuel capacity (gal.): Fuel tank capacity as tested.
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CALCULATED DATA
Power-to-weight ration (:1): Base curb weight divided by horsepower. The lower the number, the more athletic the performance.
Specific output: Measured as horsepower per liter of engine displacement. A relative indication of a vehicle’s level of
engine performance.The higher the number the higher the performance.
Brake swept area per ton: A relative quotient to gauge braking performance, the higher the number the better.
Ramp travel index: A repeatable measure of axle travel; see pages 6 and 7 for explanation.
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INTERIOR SOUND LEVELS
Idle/First gear red-line/55 mph cruise: Interior noise levels are measured with a hand-held dB meter. A comparison of a
vehicle’s ability to neutralize noise, vibration and harshness caused by engine, wind and road noise.
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